You won’t get smarter by drilling IQ tests or playing the violin. Dual nback probably won’t improve your working memory. But you can remember anything you choose to with spaced repetition.
Spaced repetition is, by far, the most effective cognitive hack I’ve used. It used to be that I’d read a book and afterwards remember almost nothing. Sometimes I’d take Kindle highlights, or notes, but never review them. And I hoped that though I could not necessarily name what I recalled (because whose memory has an index?) that somehow the important information was woven into my knowledge, tacitly. This is mostly a cope.
I love learning, and spaced repetition has helped me become extremely good at it. But it took a long time to become effective at it. There’s a lot of advice on the Internet on how to do it effectively, most of which is phrased in terms of very general principles, but very few concrete examples. But what people struggle with is: how do I turn this specific, concrete piece of information into a set of flashcards?
This post describes the rules I use to write effective flashcards, with as many examples as I could reasonably find.
\[\def\set#1{\{#1\}}\]Contents
 Overview of Spaced Repetition
 Limiting Factors
 Words of Encouragement
 Rules
 Examples
 Example: Magma Formation
 Example: Plate Tectonics
 Example: Neural Cells
 Example: Neuron Types
 Example: Vector Spaces
 Example: Parity Group
 Example: Logical Consequence
 Example: Periodization
 Example: Rational Numbers
 Example: Regular Expressions
 Example: Voltage
 Example: Isomers
 Example: Months of the Year
 Example: Powers of Two
 Example: Rilke
 Example: Pharmacology
 Example: Misc. Examples
 Scripts
 Software
 Prior Art
 Conclusion
Overview of Spaced Repetition
The idea is mechanically very simple: as you learn things, you write flashcards, which are question/answer pairs. Then you review them: you look at the question, recall the answer, and turn the card over to see the answer. Then you grade yourself: did you recall correctly or incorrectly?
If you keep recalling correctly, the review interval grows longer; if you get it wrong, the interval gets shorter.
Some people use paper cards, most people use software such as Anki, because the algorithm schedules reviews efficiently, so you don’t overreview material. Unless you have a paper fetish, just use software.
Limiting Factors
If you’re so smart, why aren’t you rich? Or: if spaced repetition is so effective, why doesn’t everyone do it? Why isn’t it as common as drinking coffee?
There are two main limiting factors to effective spaced repetition.
Habit Formation
For spaced repetition to be useful, it has to be a habit. I drill flashcards every day as part of my morning routine. But habit formation is difficult, doubly so for people who have ADHD or are low in conscientiousness.
The reason you have to do it every day is that the spaced repetition algorithm schedules the reviews for you, freeing you from having to do that manually. But you don’t know what cards are due in a given day until you open the app. And if you skip a day, those cards pile up and are due the next day.
A common failure mode (and I did this more than once, before I got the hang of it) is to use Anki for two weeks, then drop it, and pick it back up six months later only to find you have 600 cards due for review. This is not encouraging, and it defeats the point of spaced repetition, which is to review the cards on the intervals the algorithm chooses.
I don’t have much advice in this area, except that if you have persistent problems with conscientiousness, untreated ADHD etc. you should address that first.
CardWriting Skills
Writing effective flashcards is a skill that took me a while to acquire. Many of the cards I wrote in the first four or six months of using spaced repetition consistently turned out pretty much useless, and this can be frustrating. The main reason to write this post was to communicate the lessons I learnt so you can jump in to using spaced repetition effectively from the start.
One reason this can be frustrating is you’ll often remember a flashcard for the first few weeks of it (when you’re seeing it with high frequency), but after a couple of months, you start failing it. It didn’t take root in your long term memory, because it was poorly written in some way. And this long feedback cycle means it takes time to acquire these skills through trial and error.
Words of Encouragement
Learning is an automatic, instinctual process. It’s a fundamental feature of intelligence. It’s a testament to how bad schooling is that people think they have to have a special kind of brain to learn effectively, and that the idea of learning triggers aversion in people. Remember the words of Feynman: “what one fool can do, another can”.
Rules
Here are my rules for effective spaced repetition.
The rules are sorted by applicability (but not necessarily importance), with the more general ones first, and most specific ones last.
Because many of the examples involve multiple rules at the same time, I decided to list the examples separately from the list of rules.
Rule: Understand First
Don’t try to memorize what you don’t understand. The concepts should be clear in your head before you try to commit them to memory. “Clear” can be a fuzzy thing. What I tend to do is: dig, expand, and clarify the text until I’m comfortable I have a good grasp of this region of the concept graph, and then write the flashcards.
Often, when reading a book, you can’t write the flashcards exactly as you read the text, because further information can clarify or tie together important concepts. It can be useful to keep a scratchpad where you write tentative flashcard text as you read a chapter, and at the end you organize and reorganize your scratchpad until you can commit it to flashcards.
Rule: Be Honest
The software doesn’t know whether you recalled something correctly or not. You are only accountable to yourself. If you recalled something wrong, or not quite right, err on the side of caution and mark it forgotten.
Rule: Keep It Fun
This is crucial to maintaining the habit. If reviewing flashcards feels like a chore, you will become averse to doing it.
I used to frequently have this problem. I solved it in a few ways:
 Having a diverse knowledge base you’re drilling helps, so you are not bored by going through the same topic for a long time. Typically, spaced repetition software will shuffle the cards, so that if you’re drilling all the cards across all decks, you will be surprised often.
 A common source of frustration is cards that are too long to recall quickly, and thus feel like a chore. Break big cards down into smaller cards. It feels good to be able to fly through the cards quickly.
 Cards that are difficult to recall are very frustrating. I solved this by applying the rules described in this post.
Rule: Repeat Yourself
Memory is frequency times volume. Individual cards should be extremely brief, but your deck as a whole can be as repetitive as you want.
Rule: Organize by Source
Organize content by source, not topic.
The reason is you’ll often bring in information from multiple sources: multiple textbooks, plus Wikipedia, plus lecture notes, etc. Each one of these sources likely has a different way of organizing knowledge.
Don’t waste time trying to find the perfect ontology.
Make a deck for each source. In the case of textbooks, make a subdeck for each chapter. In the case of math textbooks, possibly make a subsubdeck in each chapter to put theorem cards.
This also makes it easier to keep track of how far along you got into a text.
Rule: Write Atomic Flashcards
Cards should be short. They should refer to as little information as possible. They should be like chemical bonds, linking individual atoms of knowledge.
This is the most important thing. By far the worst failure mode is to put too much in a flashcard.
There’s two reasons for this rule:
 Larger cards are harder to remember.
 It’s harder to objectively grade yourself: when you reveal the answer, you might have got some things right and some things wrong. If you click forget, you will be overreviewing the parts you already know. If you click remembered, you will underreview the parts you forgot.
There is one exception to this: you can have big cards if you also have smaller cards that add up to the same information. You can think of the larger card as testing that you can collate the information from the smaller cards.
Rule: Write TwoWay Questions
When possible, ask questions in two directions.
Whenever you have a term with a definition, the obvious thing to do is to ask for the definition from the term, e.g.:
Q: What is the order of a group?
A: The cardinality of its underlying set.
But you can also ask for the term from the definition, e.g.:
Q: What is the term for the cardinality of a group?
A: The group’s order.
When you have some notation, like $\mathbb{R}$ for the real numbers, or $\dim V$ for the dimension of a vector space, the natural thing to ask is what the notation means.
Q: What does $\mathbb{R}$ stand for?
A: The set of real numbers.
You can also ask the question backwards:
Q: What is the notation for the set of real numbers?
A: $\mathbb{R}$
Rule: Ask Questions in Multiple Ways
Ask questions in multiple ways. Ask for formal and informal definitions of terms. Ask for the formal and informal statements of a theorem. Ask questions forwards and backwards. Add contextual questions: “what is the intutition for [concept]?”. Add questions that link different concepts across your knowledge graph.
The more interlinked your knowledge graph is, the better.
Rule: Concept Graphs
It can help to visualize the concepts you’re acquiring as being like a graph, where each node represents a discrete concept having certain properties, and the edges in the graphs are questions which get you from one concept to another.
Rule: Cache Your Insights
When studying, you will often infer new knowledge that is not explicitly written down in the text by thinking about what you’ve just read. After verifying that your inference is actually true, it can be helpful to “cache your insight”—to make a flashcard for that new discovered piece of knowledge.
Rule: Learning Hierarchies
A lot of knowledge is hierarchical, of the form “Foo can be either A, B, or C”, or, dually, “A is a kind of Foo”. By analogy to OOP: these concepts are joined by superclass and subclass relations.
The idea is to ask questons in the top down direction (“What are the subclasses of Foo?”) and the bottomup direction (“What is Bar a subclass of?”).
This ties into keeping flashcards atomic. Even when some information is not hierarchical, intrinsically, breaking down large flashcards into smaller flashcards is fundamentally building a hierarchy of flashcards.
Rule: Learning Sequences
In general, to learn a sequence $(A_1, \dots, A_n)$, you want to generate the following flashcards for each $i \in [1,n]$:
Question  Answer 

What is the $i$th element?  $A_i$ 
What is the position of $A_i$?  $i$ 
What element comes after $A_i$?  $A_{i+1}$ 
What element comes before $A_i$?  $A_{i1}$ 
You might also want:
 A test card: a flashcard asking you to recite the sequence from beginning to end.
 A cloze sequence: flashcard with a cloze deletion for each element in the sequence, to fill in the blank given the context.
The sequence script can generate these for you.
How thorough you want to be depends on the nature of the information. Most of the time I use a cloze card and a test card.
Another type of card you might use (I use this to memorize poems) is a card that gives you some context (the previous one or two items in the sequence) and asks you to fill in the blank. For example, if you wanted to learn the sequence (A, B, C, D), you might have these flashcards:
Question  Answer 

Beginning, …  A 
Beginning, A, …  B 
A, B, …  C 
B, C, …  D 
The poetry script can generate these for you.
Examples
Many of these examples are overkill: we collect a lot more flashcards than the subject deserves. But this is to illustrate the general rules. With experience, you can learn how many questions a particular topic requires, and different volumes of your knowledge graph will be more or less interlinked.
Example: Magma Formation
From my geology notes:
Magma is liquid rock under the Earth’s surface.
The three magmaforming processes are:
 Increasing Temperature: increasing temperature can melt rock.
 Decreasing Pressure: when the pressure drops, atoms are more free to move, and rock becomes liquid.
 Addition of Water: water lowers the melting point of rock, because the water molecules disrupt the crystal bonds.
Magma forms in three places:
 Hot spots: as hot rock rises, pressure decreases and it becomes magma.
 Rift zones: as tectonic plates are pulled apart, hot rock rises (because it is less dense) to plug the gap and melts due to decreasing pressure.
 Subduction zones: waterrich ocean lithosphere sinks into the mantle. The water heats up and rises, adding water to the overlying rock, which the melts.
Let’s break this down hierarchically. We want to memorize three things:
 What magma is.
 How it forms.
 Where it forms.
First, the definition:
Question  Answer 

What is magma?  Liquid rock under the Earth’s surface. 
What is the term for liquid rock under surface of the Earth?  Magma. 
Second, we want to know how magma forms. A common mistake here would be to put the magmaforming processes and their explanations in the same flashcard. Rather, to keep each card as small as possible, we want to separate the list of processes from their definitions.
So we first as for a list of mechanisms:
Question  Answer 

What are the magmaforming processes?  Increasing temperature, decreasing pressure, addition of water. 
And then we ask for an explanation of each. We don’t really need to ask why adding temperature melts rock:
Question  Answer 

Why does decreasing pressure melt rock?  Because the atoms are more free to move. 
Why does adding water lower the melting point of rock?  Because water molecules disrupt the bonds in the rock minerals. 
Third: where magma is found. Again, we separate the list from the details:
Question  Answer 

Where does magma form?  Over hot spots, in rift zones, and in subduction zones. 
Then we ask for details. For each place where magma forms, we ask both which processes are involved, and what the full causal explanation is. We also ask the question backward: which places involve a given process.
Question  Answer 

What magmaforming process happens over a hot spot?  Pressurerelease melting. 
What magmaforming process happens in a rift zone?  Pressurerelease melting. 
What magmaforming process happens in a subduction zone?  Increasing temperature and addition of water. 
Where does magma form due to pressure release?  Hot spots and rift zones. 
Where does magma form due to increasing temperature and the addition of water?  Subduction zones. 
How does magma form in a hot spot?  As hot mantle rock rises, the decrease in pressure causes it to melt. 
How does magma form in a rift zone?  As the tectonic plates move apart, hot rock rises to fill the gap, and the decrease in pressure causes it to melt. 
How does magma form in a subduction zone?  Waterlogged crust dives into the mantle, the water turns to steam and rises, the addition of water to overlying rock causes it to melt. 
We can visualize the resulting knowledge graph like this:
Example: Plate Tectonics
Here’s the information:
The zone where two or more tectonic plates meet is called a plate boundary. There are three kinds:
 Convergent boundaries: plates come together.
 Divergent boundaries: plates move apart.
 Transform boundaries: plates slide past each other.
Applying the rule that cards should be twoway, we want two flashcards for the term ‘plate boundary’.
Question  Answer 

What is a plate boundary?  The place where tectonic plates meet. 
What is the term for the place where tectonic plates meet?  Plate boundary. 
For the different types of plate boundary, we only ask the question in the topdown direction (we don’t need to ask “what kind of thing is a transform boundary?”, since the name kind of gives it away):
Question  Answer 

What are the types of plate boundary?  Convergent, divergent, transform. 
For each kind of plate boundary, we also ask the question in two ways:
Question  Answer 

Definition: convergent boundary.  Where tectonic plates come together. 
Definition: divergent boundary.  Where tectonic plates move apart. 
Definition: transform boundary.  Where tectonic plates slide past each other. 
Term: place where tectonic plates come together.  Convergent boundary. 
Term: place where tectonic plates move apart.  Divergent boundary. 
Term: place where tectonic plates slide past each other.  Transform boundary. 
Graphically, here’s how the questions link the concepts in the knowledge graph:
Example: Neural Cells
Cells in the nervous system are divided into neurons and glia. Glial cells are divided into macroglia and microglia. Macroglia are divided into astrocytes, oligodendrocytes, and Schwann cells.
Visually:
We first write the topdown questions:
Question  Answer 

What kinds of cell make up the nervous system?  Neurons and glia. 
What are the kinds of glial cell?  Microglia and macroglia. 
What are the kinds of macroglia?  Astrocytes, oligodendrocytes, and Schwann cells. 
And the bottomup questions. We don’t ask these when the answers are obvious: “what are microglia/macroglia a kind of” has an obvious answer.
Question  Answer 

Astrocytes are a kind of…  Macroglia. 
Oligodendrocytes are a kind of…  Macroglia. 
Schwann cells are a kind of…  Macroglia. 
Example: Neuron Types
This is a brief example about keeping cards short and using hierarchies to break things down.
From my neuroscience notes:
Neurons can be divided into three categories by their function:
 Sensory: feed sensory information into the brain.
 Motor: send motor commands to the muscles.
 Interneurons: connect within the CNS. These are further divided into:
 Local: form circuits with nearby neurons.
 Relay: have long axons and communicate across brain regions.
Let’s start by doing this the wrong way, by loading too much information into one card.
Question  Answer 

What are the functional categories of neuron? 

What are the different types of interneuron? 

Let’s first break this down by separating terms from definitions:
Question  Answer 

What are the functional categories of neuron?  Sensory, motor, interneurons. 
What are sensory neurons?  Neurons which feed information into the brain. 
What are motor neurons?  Neurons which send commands to the muscles. 
What are interneurons?  Neurons which connect within the CNS. 
What are the types of interneuron?  Local, relay. 
What are local interneurons?  Interneurons that form circuits with nearby neurons. 
What are relay interneurons?  Interneurons have long axons and communicate across brain regions. 
Now we ask the questions in the backward direction: from the definition to the term:
Question  Answer 

What is the term for a neuron that feeds information into the brain?  Sensory neuron. 
What is the term for a neuron that sends commands to the muscles?  Motor neuron. 
What is the term for a neuron that connects within the CNS?  Interneuron. 
What is the term for an interneuron that forms circuits with nearby neurons?  Local interneuron. 
What is the term for an interneuron that communicates across brain regions?  Relay interneuron. 
Example: Vector Spaces
Here’s what we want to learn:
A vector space, informally, is a set whose elements—called vectors—can be added or scaled.
More formally: a vector space over a field $\mathbb{F}$ is a set $V$ plus two operations:
 Vector addition: $V \times V \to V$
 Scalar multiplication: $V \times \mathbb{F} \to V$
Satisfying the following axioms:
 Commutativity of Addition
 $u + v = v + u$
 Associativity of Addition
 $u + (v + w) = (u + v) + w$
 Identity of Addition
 $\exists 0 \in V : v + 0 = v$
 Inverse of Addition
 $\forall v \in V, \exists v \in V : v + (v) = 0$
 Identity of Scaling
 $1v = v$
 Distributivity
 $\forall v \in V, a,b \in \mathbb{F} : (a+b)v = av + bv$
We have to break this down. Severely. We will do this step by step.
First, we have to separate the informal (intuitive) and formal definitions:
Question  Answer 

Informally: what is a vector space?  A set whose elements can be added or scaled. 
Formally: what is a vector space?  A vector space over a field $\mathbb{F}$ is a set $V$ plus two operations: vector addition and scalar multiplication. 
We add one brief question about notation (you may choose to skip this one, it’s an example):
Question  Answer 

What are the elements of a vector space called?  Vectors. 
Now we ask about the signatures of the operations:
Question  Answer 

What is the signature of vector addition?  $V \times V \to V$ 
What is the signature of scalar multiplication?  $V \times \mathbb{F} \to V$ 
Next, we ask for the axioms:
Question  Answer 

What are the axioms that define a vector space? 

Finally, we ask what each axiom means:
Question  Answer 

Vector spaces: state: commutativity of addition  $u + v = v + u$ 
Vector spaces: state: associativity of addition  $u + (v + w) = (u + v) + w$ 
Vector spaces: state: identity of addition  $\exists 0 \in V: v + 0 = v$ 
Vector spaces: state: inverse of addition  $\forall v \in V, \exists v \in V: v + (v) = 0$ 
Vector spaces: state: identity of scaling  $1v = v$ 
Vector spaces: state: distributivity  $\forall v \in V, a,b \in \mathbb{F}: (a+b)v = av + bv$ 
Graphically, you can try visualizing the flashcards and their relationships like this:
If you want to be extra thorough, you can also write the backwards questions:
Question  Answer 

What is the term for a set whose elements can be added or scaled?  A vector space. 
Name this axiom: $u + v = v + u$  Commutativity of Addition 
Name this axiom: $u + (v + w) = (u + v) + w$  Associativity of Addition 
Name this axiom: $\exists 0 \in V : v + 0 = v$  Identity of Addition 
Name this axiom: $\forall v \in V, \exists v \in V : v + (v) = 0$  Inverse of Addition 
Name this axiom: $1v = v$  Identity of Scaling 
Name this axiom: $\forall v \in V, a,b \in \mathbb{F} : (a+b)v = av + bv$  Distributivity 
Example: Parity Group
The parity group is a group that represents the rules for adding even and odd numbers. The underlying set is $\set{\text{even}, \text{odd}}$, with $\text{even}$ and $\text{odd}$ representing even and odd numbers respectively. The composition table is:
$+$  $\text{even}$  $\text{odd}$ 
$\text{even}$  $\text{even}$  $\text{odd}$ 
$\text{odd}$  $\text{odd}$  $\text{even}$ 
The identity element is $\text{even}$. The group is Abelian.
We can turn this into the following flashcards:
Question  Answer 

What is the parity group?  The group that represents the rules for adding even and odd numbers. 
What is the order of the parity group?  $2$ 
What is the underlying set of the parity group?  $\set{\text{even}, \text{odd}}$ 
What is the identity element of the parity group?  $\text{even}$ 
What is the operation of the parity group?  Addition of even and odd numbers. 
$\text{even} + \text{even} = $  $\text{even}$ 
$\text{even} + \text{odd} = $  $\text{odd}$ 
$\text{odd} + \text{even} = $  $\text{odd}$ 
$\text{odd} + \text{odd} = $  $\text{even}$ 
Is the parity group Abelian? Why or why not?  Yes, because addition commutes. 
Example: Logical Consequence
From my notes on logic:
The two notions of logical consequence are:
 Semantic Consequence: $Q$ is a semantic consequence of $P$ iff, in every interpretation where $P$ is true, $Q$ is also true. This is denoted $P \models Q$.
 Syntactic Consequence: $Q$ is a syntactic consequence of $P$ iff there exists a proof from $P$ to $Q$. This is denoted $P \vdash Q$.
Semantic consequence is about interpretations, while syntactic consequence is about proofs.
We begin with the most basic question:
Question  Answer 

What are the two notions of logical consequence?  Semantic and syntactic. 
Then we ask questions specifically about semantic consequence:
Question  Answer 

Define semantic consequence  $Q$ is a semantic consequence of $P$ iff in every interpretation where $P$ is true, $Q$ is also true. 
What’s the notation for “$Q$ is a semantic consequence of $P$”?  $P \models Q$ 
What does $P \models Q$ mean?  $Q$ is a semantic consequence of $P$ 
Semantic consequence connects sentences by…  Interpretations. 
Which notion of logical consequence involves interprerations?  Semantic consequence. 
And then about syntactic consequence:
Question  Answer 

Define syntactic consequence  $Q$ is a syntactic consequence of $P$ iff there is a proof from $P$ to $Q$. 
What’s the notation for “$Q$ is a syntactic consequence of $P$”?  $P \vdash Q$ 
What does $P \vdash Q$ mean?  $Q$ is a syntactic consequence of $P$ 
Syntactic consequence connects sentences by…  Proofs. 
Which notion of logical consequence involves proofs?  Syntactic consequence. 
Example: Periodization
Timelines are a great example of how breaking information down hierarchically can help you learn long sequences. Sometimes the breakdown is already done for us.
The geologic time scale (GTS) divides the geological record of the Earth into four nested time units:
 The eon is the largest unit. Eons last hundreds of millions of years.
 Eons are further divided into eras, which last tens to hundreds of millions of years.
 Eras are divided into periods, which last millions to tens of millions of years.
 Finally, periods are divided into epochs, which last hundreds of thousands to millions of years.
The four eons, from oldest to most recent, are:
 Hadean (4.5Gya to 4Gya)
 Archean (4Gya to 2.5Gya)
 Proterozoic (2.5Gya to 538Mya)
 Phanerozoic (538Mya to present)
We want to learn the following things:
 What the geologic time scale is.
 How it divides the Earth’s history.
 The four eons.
Let’s begin with the simplest flashcards, the definition of the GTS:
Question  Answer 

What is the geologic time scale?  The timeline of Earth’s history. 
What is the term for the timeline of Earth’s history?  The geologic time scale. 
The subdivisions form a sequence, from oldest to most recent: eon, era, period, epoch. So let’s feed it into the sequence script. Here’s the input:
Geologic Time Units
Eon
Era
Period
Epoch
Running cat units.txt  ./sequence.py > units.csv
and importing units.csv
into Mochi, we get these flashcards:
Question  Answer 

Geologic Time Units: Recall all elements of the sequence  Eon, Era, Period, Epoch. 
Geologic Time Units: What element has position 1?  Eon. 
Geologic Time Units: What element has position 2?  Era. 
Geologic Time Units: What element has position 3?  Period. 
Geologic Time Units: What element has position 4?  Epoch. 
Geologic Time Units: What is the position of Eon?  1. 
Geologic Time Units: What is the position of Era?  2. 
Geologic Time Units: What is the position of Period?  3. 
Geologic Time Units: What is the position of Epoch?  4. 
Geologic Time Units: What comes after Eon?  Era. 
Geologic Time Units: What comes after Era?  Period. 
Geologic Time Units: What comes after Period?  Epoch. 
Geologic Time Units: What comes before Era?  Eon. 
Geologic Time Units: What comes before Period?  Era. 
Geologic Time Units: What comes before Epoch?  Period. 
You probably don’t need all of these. You can probably get away with just these:
Question  Answer 

What are the units of the geologic time scale, from largest to smallest?  Eon, era period, epoch. 
What is the largest unit in the geologic time scale?  The eon. 
What is the secondlargest unit in the geologic time scale?  The era. 
What is the thirdlargest unit in the geologic time scale?  The period. 
What is the smallest unit in the geologic time scale?  The epoch. 
Now, since this is a concept hierarchy, we also ask the “what is” questions.
Question  Answer 

What is an eon?  A division of the geologic time scale. 
What is an era?  A division of the geologic time scale. 
What is an period?  A division of the geologic time scale. 
What is an epoch?  A division of the geologic time scale. 
And, since units have a duration, we ask what for the duration. We do this forwards and backwards:
Question  Answer 

What is the duration of an eon?  Hundreds of millions of years. 
Which geologic unit lasts hundreds of millions of years?  Eons. 
What is the duration of an era?  Tens to hundreds of millions of years. 
Which geologic unit lasts tens to hundreds of millions of years?  Eras. 
What is the duration of a period?  Millions to tens of millions of years. 
Which geologic unit lasts millions to tens of millions of years?  Periods. 
What is the duration of an epoch?  Hundreds of thousands to millions of years. 
Which geologic unit lasts hundreds of thousands to millions of years?  Epochs. 
Now, the four eons. These form a sequence, we don’t do the whole sequence script thing again, since you have probably, again, just use these:
Question  Answer 

List eons from oldest to newest  Hadean, Archean, Proterozoic, Phanerozoic. 
What is the first eon?  Hadean 
What is the second eon?  Archean 
What is the third eon?  Proterozoic 
What is the fourth eon?  Phanerozoic 
We also ask when each eon began and ended, forwards and backwards:
Question  Answer 

When did the Hadean begin?  4.5 Gya 
When did the Hadean end?  4 Gya 
Which eon began 4.5 Gya?  Hadean 
Which eon ended 4 Gya?  Hadean 
When did the Archean begin?  4 Gya 
When did the Archean end?  2.5 Gya 
Which eon began 4 Gya?  Archean 
Which eon ended 2.5 Gya?  Archean 
When did the Proterozoic begin?  2.5 Gya 
When did the Proterozoic end?  538 Mya 
Which eon began 2.5 Gya?  Proterozoic 
Which eon ended 538 Mya?  Proterozoic 
When did the Phanerozoic begin?  538 Mya 
When did the Phanerozoic end?  Present 
Which eon began 538 Mya?  Phanerozoic 
Which eon is ongoing?  Phanerozoic 
Example: Rational Numbers
Let’s commit this to spaced repetition:
The set of rational numbers, denoted $\mathbb{Q}$, is the set of fractions with integer numerator and denominator, where the denominator is nonzero.
Formally:
\[\mathbb{Q} = \left\{\, \frac{p}{q} \,\, \middle \,\, p, q \in \Z \land q \neq 0 \,\right\}\]The $\mathbb{Q}$ stands for quotient.
Let’s visualize the concept graph as we build up the flashcards. We start with the central node, the concept of the rational numbers:
Then we add a notation node, linked by two forward and backwards questions:
Question  Answer 

What is the notation for the set of rational numbers?  $\mathbb{Q}$. 
What does $\mathbb{Q}$ stand for?  The set of rational numbers. 
Formal as well as informal definitions:
Question  Answer 

Informally, what is the set of rational numbers?  The set of fractions with integer numerator and denominator, where the denominator is nonzero. 
Formally, what is the set of rational numbers?  $\mathbb{Q} = \left\{\, \frac{p}{q} \,\, \middle \,\, p, q \in \Z \land q \neq 0 \,\right\}$ 
What's the term for the set of integer fractions?  The rational numbers. 
What is the name of this set? $\left\{\, \frac{p}{q} \,\, \middle \,\, p, q \in \Z \land q \neq 0 \,\right\}$  The rational numbers. 
And a final note on notation: what the $\mathbb{Q}$ stands for:
Question  Answer 

What are the rational numbers denoted by $\mathbb{Q}$?  Q for quotient. 
Example: Regular Expressions
This is an example about asking questions in two ways.
These cards go from a concept to a regex:
Question  Answer 

What regex matches the start of a line?  ^ 
What regex matches the end of a line?  $ 
What regex matches a digit?  \d 
In addition to the above, add cards that go from the regex to the concept:
Question  Answer 

What does ^ match? 
The start of a line. 
What does $ match? 
The end of a line. 
What does \d match? 
A digit 09. 
Example: Voltage
This is an example of asking questions in different ways
The voltage between two points $A$ and $B$ can be defined as either: Voltage can be defined as:
 The difference in electric potential between the two points.
 The amount of work done by a $1C$ particle as it travels from $A$ to $B$.
The idea here is:
 We first ask for the definition of voltage in terms of both electric potential and work.
 We also ask what is the term for each definition.
Which gives us the following flashcards:
Question  Answer 

What is voltage in terms of electric potential?  Difference in electric potential between two points. 
What is voltage in terms of work?  Work done by a 1C particle as it travels from A to B. 
What is the term for the difference in electric potential between two points?  Voltage. 
What is the term for the work done by a 1C particle as it travels between two points?  Voltage. 
Example: Isomers
Two chemical compounds are said to be isomers of each other if they have the same chemical formula (same number of atoms of each element) but their threedimensional structure differs.
Isomers can be divided into:
 Structural Isomers: the chemical formula is the same but the atoms are bonded differently.
 Stereoisomers: the chemical formula and the bonds are the same but the
spatial arrangement is different. These are divided into:
 Conformational Isomers: can be intercoverted by rotating about a sigma bond.
 Configurational Isomers: cannot be interconverted without breaking a
bond. These are further subdivided into:
 Enantiomers: nonsuperposable mirror images. Also called optical isomers because of the way they reflect planepolarized light.
 Diastereomers: not enantiomers. One important subtype is:
 Cis/Trans Isomers: occur when two functional groups can find themselves on the same or opposite sides of a rigid structure. When both functional groups are on the same side of the rigid structure, that is a cis isomer; when they are on opposite sides, that is a trans isomer.
This is an example of a cis isomer:
This is an example of a trans isomer:
This is fairly straightforward: we have to learn a hierarchy of definitions. We’ll divide this into two tasks:
 First, definitions. Ask questions from the term to the definition and from the definition to the term.
 Second, hierarchy: ask about subtypes and supertypes.
So let’s begin with the definitions. First we ask the questions in the forward direction:
Question  Answer 

What is an isomer?  Two compounds are isomers when they have the same chemical formula but different 3D structures. 
What are structural isomers?  Compounds with the same formula but the atoms have a different bond graph.. 
What are stereoisomers?  Compounds with the same formula and bond graph but different spatial arrangement. 
What are conformational isomers?  Isomers that can be interconverted by rotating about a sigma bond. 
What are configurational isomers?  Isomers that cannot be interconverted without breaking a bond. 
What are enantiomers?  Nonsuperposable mirror images. 
What are diastereomers?  Stereoisomers that are not enantiomers. 
What are cis/trans isomers?  Isomers where two functional groups are on the same or opposite sides of a rigid structure. 
And now the definitions, in the backward direction:
Question  Answer 

What is the term for compounds with the same chemical formula but different 3D structures?  Isomer 
What is the term for isomers with the same formula but a different bond graph?  Structural Isomer 
What is the term for isomers that have the same bond graph different spatial arrangement?  Stereoisomer 
What is the term for isomers that can be interconverted by rotating about a sigma bond?  Conformational Isomer 
What is the term for isomers that cannot be interconverted without breaking a bond?  Configurational Isomer 
What is the term for nonsuperposable mirror images?  Enantiomer 
What is the term for stereoisomers that are not enantiomers?  Diastereomer 
What is the term for isomers where two functional groups are on the same or opposite sides of a rigid structure?  Cis/Trans Isomer 
We left some information out, to keep the cards atomic, now we have to ask questions to recall that information:
Question  Answer 

What is another term for enantiomer?  Optical isomer. 
Why are enantiomers also called optical isomers?  Because of the way they reflect planepolarized light. 
What is an optical isomer?  Another term for enantiomer. 
What is a cis isomer?  One with both functional groups on the same side of a rigid structure. 
What is the term for an isomer with both functional groups on the same side a rigid structure.  A cis isomer. 
What is a trans isomer?  One with both functional groups on the same side of a rigid structure. 
What is the term for an isomer with both functional groups on the same side a rigid structure.  A trans isomer. 
Now we move on to the hierarchy, which connects these concepts. We first ask the questions in the downward direction, from parent to child:
Question  Answer 

What are the subtypes of isomers?  Structural isomers, stereoisomers. 
What are the subtypes of stereoisomers?  Conformational isomers, configurational isomers. 
What are the subtypes of configurational isomers?  Enantiomers, diastereomers. 
What are the subtypes of diastereomers?  Cis/trans isomers. 
And now in the upward direction:
Question  Answer 

Structural isomers are a kind of …  Isomer 
Stereoisomers are a kind of …  Isomer 
Conformational isomers are a kind of …  Stereoisomer 
Configurational isomers are a kind of …  Stereoisomer 
Enantiomers are a kind of …  Configurational isomer 
Diastereomers are a kind of …  Configurational isomer 
Cis/trans isomers are a kind of …  Diastereomer 
And, finally, the examples:
Question  Answer 

What kind of isomer is this?  A cis isomer. 
What kind of isomer is this?  A trans isomer. 
Example: Months of the Year
Suppose you want to memorize:
 January
 February
 March
 April
 May
 June
 July
 August
 September
 October
 November
 December
The indextoelement flashcards:
Question  Answer 

What is the 1st month of the year?  January 
What is the 2nd month of the year?  February 
What is the 3rd month of the year?  March 
What is the 4th month of the year?  April 
What is the 5th month of the year?  May 
What is the 6th month of the year?  June 
What is the 7th month of the year?  July 
What is the 8th month of the year?  August 
What is the 9th month of the year?  September 
What is the 10th month of the year?  October 
What is the 11th month of the year?  November 
What is the 12th month of the year?  December 
The elementtoindex flashcards:
Question  Answer 

January is the … month of the year.  1 
February is the … month of the year.  2 
March is the … month of the year.  3 
April is the … month of the year.  4 
May is the … month of the year.  5 
June is the … month of the year.  6 
July is the … month of the year.  7 
August is the … month of the year.  8 
September is the … month of the year.  9 
October is the … month of the year.  10 
November is the … month of the year.  11 
December is the … month of the year.  12 
The successor flashcards:
Question  Answer 

What comes after January?  February 
What comes after February?  March 
What comes after March?  April 
What comes after April?  May 
What comes after May?  June 
What comes after June?  July 
What comes after July?  August 
What comes after August?  September 
What comes after September?  October 
What comes after October?  November 
What comes after November?  December 
And the predecessor flashcards:
Question  Answer 

What comes before February?  January 
What comes before March?  February 
What comes before April?  March 
What comes before May?  April 
What comes before June?  May 
What comes before July?  June 
What comes before August?  July 
What comes before September?  August 
What comes before October?  September 
What comes before November?  October 
What comes before December?  November 
Example: Powers of Two
Let’s memorize the first sixteen powers of two:
The forward cards ask for the power:
Question  Answer 

$2^2$  $4$ 
$2^3$  $8$ 
$2^4$  $16$ 
$2^5$  $32$ 
$2^6$  $64$ 
$2^7$  $128$ 
$2^8$  $256$ 
$2^9$  $512$ 
$2^{10}$  $1024$ 
$2^{11}$  $2048$ 
$2^{12}$  $4096$ 
$2^{13}$  $8192$ 
$2^{14}$  $16384$ 
$2^{15}$  $32768$ 
$2^{16}$  $65536$ 
While the backwards cards ask for the exponent from the power:
Question  Answer 

$\log_2 4$  $2$ 
$\log_2 8$  $3$ 
$\log_2 16$  $4$ 
$\log_2 32$  $5$ 
$\log_2 64$  $6$ 
$\log_2 128$  $7$ 
$\log_2 256$  $8$ 
$\log_2 512$  $9$ 
$\log_2 1024$  $10$ 
$\log_2 2048$  $11$ 
$\log_2 4096$  $12$ 
$\log_2 8192$  $13$ 
$\log_2 16384$  $14$ 
$\log_2 32768$  $15$ 
$\log_2 65536$  $16$ 
Finally, I have a test card that asks me to recall the entire sequence in order.
Example: Rilke
Let’s memorize this poem:
Archaic Torso of Apollo
Rainer Maria Rilke
We cannot know his legendary head
with eyes like ripening fruit. And yet his torso
is still suffused with brilliance from inside,
like a lamp, in which his gaze, now turned to low,
gleams in all its power. Otherwise
the curved breast could not dazzle you so, nor could
a smile run through the placid hips and thighs
to that dark center where procreation flared.
Otherwise this stone would seem defaced
beneath the translucent cascade of the shoulders
and would not glisten like a wild beast’s fur:
would not, from all the borders of itself,
burst like a star: for here there is no place
that does not see you. You must change your life.
You could run this through the sequence script, but that seems a bit cold and mechanical. We’ll use the poetry script instead, which shows us two lines of context and asks us to complete the next line. The generated flashcards are:
Question  Answer 

Beginning ... 
We cannot know his legendary head 
Beginning We cannot know his legendary head ... 
with eyes like ripening fruit. And yet his torso 
We cannot know his legendary head with eyes like ripening fruit. And yet his torso ... 
is still suffused with brilliance from inside, 
with eyes like ripening fruit. And yet his torso is still suffused with brilliance from inside, ... 
like a lamp, in which his gaze, now turned to low, 
is still suffused with brilliance from inside, like a lamp, in which his gaze, now turned to low, ... 
gleams in all its power. Otherwise 
And so on. You get the pattern.
Example: Pharmacology
The dissociation constant ($K_d$) of a drug is the drug concentration where half the binding sites in an assay are occupied.
This will be an example of caching your insights. From this text, we can deduce more things:
 A high value of $K_d$ means the drug has a low affinity for the binding sites, because it takes a higher concentration to reach the same amount of binding.
 A low value of $K_d$ means the drug has a high affinity for the binding sites, because it takes a lower concentration to reach the same occupancy.
And from the above two facts we can also conclude:
 $K_d$ is inversely proportional to binding affinity.
From this, we can start asking the questions:
Question  Answer 

What is the term for the drug concentration where half the binding sites are occupied?  Dissociation constant. 
What is the notation for the dissociation constant?  $K_d$ 
What does $K_d$ stand for?  The dissociation constant. 
What does a low value of $K_d$ mean?  High binding affinity. 
Why does a low value of $K_d$ imply high binding affinity?  Fewer molecules are needed to reach the same occupancy. 
What does a high value of $K_d$ mean?  Low binding affinity. 
Why does a high value of $K_d$ imply low binding affinity?  More molecules are needed to reach the same occupancy. 
If a drug’s binding affinity is high, what does that tell us about $K_d$?  $K_d$ is low. 
If a drug’s binding affinity is low, what does that tell us about $K_d$?  $K_d$ is high. 
Describe the relationship between $K_d$ and binding affinity.  $K_d$ is inversely proportional to binding affinity. 
$K_d$ is ___ proportional to binding affinity.  Inversely. 
Visually:
Example: Misc. Examples
Given:
US Treasury bonds are called treasuries.
You can write the following forwardbackward cards:
Question  Answer 

What are US Treasury bonds nicknamed?  Treasuries. 
What is nicknamed ‘treasuries’?  US Treasury bonds. 
Given:
The derivative of $\sin x$ is $\cos x$.
You can ask for both the derivative and the antiderivative:
Question  Answer 

Derivative: $\sin x$  $\cos x$ 
Integral: $\cos x$  $\sin x$ 
Scripts
A lot of these rules would be impossible to apply if we had to write all the flashcards by hand: it would simply be too tiresome. Fortunately we have automation.
I put up the scripts I use in this repository. These are largely based on Gwern’s scripts.
Sequence Script
The sequence.py
script generates flashcards for learning a sequence, according
to the principles in Rule: Learning Sequences.
Given a file greek.txt
as input:
Greek Alphabet
Alpha
Beta
Gamma
We can run cat greek.txt  ./sequence.py > cards.csv
, and this will generate the following:
Question  Answer 

Greek Alphabet: Recall all elements of the sequence  Alpha, Beta, Gamma. 
Greek Alphabet: What element has position 1?  Alpha. 
Greek Alphabet: What element has position 2?  Beta. 
Greek Alphabet: What element has position 3?  Gamma. 
Greek Alphabet: What is the position of Alpha?  1. 
Greek Alphabet: What is the position of Beta?  2. 
Greek Alphabet: What is the position of Gamma?  3. 
Greek Alphabet: What comes after Alpha?  Beta. 
Greek Alphabet: What comes after Beta?  Gamma. 
Greek Alphabet: What comes before Beta?  Alpha. 
Greek Alphabet: What comes before Gamma?  Beta. 
Plus the cloze card:
Cloze 

Greek Alphabet: Elements of the sequence: [[Alpha]], [[Beta]], [[Gamma]]. 
Poetry Script
The poetry.py
script generates flashcards where you are given two lines of a
poem and have to recall the next line. For an example of using this, see
here.
Software
Most people use Anki. It’s open source, has lots of plugins, and has a free card sync service. This is the default choice.
I use Mochi. It has a much nicer UI than Anki, which helps in maintaining the habit because it’s more pleasant to use. It also supports cloze deletion for images out of the box, as opposed to relying on a plugin.
The only Anki feature I care about that Mochi lacks is note types. Anki’s note types let you generate multiple cards from the same structured information. For example, you could have a “Chemical Element” note type with fields like “Name”, “Symbol”, “Atomic Number”, and card templates like:
Question  Answer 

What’s the symbol of {name}?  {symbol} 
What’s the atomic number of {name}?  {z} 
What element has the symbol {symbol}?  {name} 
What element has atomic number {z}?  {name} 
So from one note (“Hafnium”, “Hf”, “72”) you automatically derive four flashcards. And if you edit the note data, all the cards get updated. Mochi lacks this feature, in practice it’s not terrible, and you can get most of the way there with cloze deletions.
Prior Art
The most cited resource on writing flashcards is a classic 1999 article by Piotr Woźniak, creator of SuperMemo. The article is Effective learning: twenty rules of formulating knowledge and I endorse most of it. Let me reiterate the important parts:
 Understand before memorizing: the concepts have to be clear in your head before you try to commit them to memory. “Clarity” is a fuzzy thing, but what I do is dig, expand, clarify until I feel comfortable formulating knowledge.
 Cards should be atomic, and should refer to the smallest possible conceptual units.
 Avoid trying to memorize long sequences.
 Avoid trying to memorize big unordered sets of things.
 Keep the wording simple.
 Redundancy is good. Do repeat yourself!
Conclusion
I hope that you will find these rules and examples useful. Now go pick up a textbook and learn something useful.