The Leitner system is a way of running flashcards so the cards you keep getting wrong come back sooner, and the cards you already know drift further and further away until you barely see them. That's it. No app required to start, no special stationery, no clever trick: just a few boxes (or a few labelled piles) and one honest rule about where each card goes next. It was invented by a German journalist called Sebastian Leitner in the 1970s, and the reason it's still around is that it quietly does the two things revision actually needs: it makes you pull answers out of your head, and it spaces those attempts out over time.
What the Leitner system actually is
Picture five boxes in a row, numbered 1 to 5. Every flashcard starts in Box 1. When you test yourself on a card and get it right, it moves up one box. When you get it wrong, it goes all the way back to Box 1, no matter which box it was in. The catch is how often you review each box: Box 1 gets looked at most often, Box 5 barely ever. So a card you keep fumbling stays in the fast lane, showing up again and again, while a card you've genuinely nailed earns its way to the slow lane and stops eating your time.
The whole thing is self-correcting. You're not deciding what to revise based on a vague feeling of "I should probably go over cells again." The boxes decide for you, based on what you actually got right and wrong. That's the bit that makes it better than flipping through a stack in order. The stack treats every card as equally important, and they're not.
How to set up your boxes
You don't need actual boxes. Five elastic-banded piles on your desk work fine, or five sections in a card wallet, or five dividers in an index-card tin. Label them 1 to 5. Write your cards (question on one side, answer on the other) and drop the whole lot into Box 1 to begin with.
Then set a rough review rhythm. A version that works for a GCSE term looks like this: review Box 1 every day, Box 2 every two days, Box 3 twice a week, Box 4 once a week, Box 5 once a fortnight. You don't have to be precise about it. The point isn't the exact numbers. It's that each box up the chain gets reviewed less often than the one below it. If keeping five boxes feels like a lot, run three: daily, every few days, weekly. Three boxes is still miles better than no system.
The rules that make it work
There are only two, and both are about being honest with yourself. First: a card only moves up if you got it right before flipping it over. Not "right-ish." Not "I would've got that." If you had to peek, it stays where it is or goes back to Box 1. Second: a wrong card always returns to Box 1, even if it had climbed all the way to Box 4. That feels harsh the first time it happens, but it's the whole engine. A card you thought you knew and then blanked on is exactly the card you need to see more often, not less.
The temptation is to be generous with yourself because moving cards up feels like progress. Resist it. The system is only useful if the boxes reflect what's really in your head, not what you'd like to be there.
Why it beats going through every card every time
Two things are doing the heavy lifting here, and they're both backed by decades of research on how memory works. The first is retrieval practice: the act of trying to recall an answer before you check it. Pulling something out of your memory strengthens it far more than reading it again does. In one well-known set of experiments, students who tested themselves on material remembered dramatically more a week later than students who simply re-read it, even though the re-readers felt more confident at the time (Roediger & Karpicke, 2006). Flashcards done as a real test (answer first, flip second) are retrieval practice in its purest form.
The second is spacing. Reviewing something a few times spread across days beats cramming the same number of reviews into one sitting. Leaving a gap lets you very slightly forget, and the small effort of hauling the answer back is what builds a durable memory (Dunlosky et al., 2013, rated both practice testing and distributed practice as the two highest-utility study techniques they reviewed). The Leitner boxes bake spacing in automatically: higher boxes simply are longer gaps. You get the benefit without having to plan a single interval yourself.
Making cards that are actually worth reviewing
A Leitner box is only as good as the cards in it. Keep each card to one idea. "The heart" is too big for a card; "Which side of the heart pumps blood to the lungs?" is the right size. Write the question so there's a single, checkable answer. Vague prompts let you off the hook, and the whole method depends on a clean right-or-wrong call. Where your spec uses specific wording, put that wording on the card, because that's what the mark scheme is looking for. And make your own cards where you can; the act of turning a topic into a question is itself a chunk of revision.
Where an app fits in
Paper boxes are great until you're revising ten subjects and the piles take over your desk. This is the part a phone quietly handles well. GCSE Boss works on the same principle as a Leitner box: it cycles through the spec points for your exact board, brings back the ones you got wrong sooner, and spaces out the ones you've shown you know, so a card only "graduates" once you've recalled it correctly across spaced attempts. You get the box method without carrying the boxes, and it's tied to what's actually on your syllabus rather than a generic question bank. If you'd rather keep it on paper, keep it on paper. The method matters more than the medium.
The honest version
The Leitner system won't make revision feel effortless, and it isn't meant to. Getting a card wrong and sending it back to Box 1 is the point, not a failure. That little sting of missing one is your memory getting the workout it needs. Start small: pick one subject, make twenty cards, run three boxes for a week, and see how it feels. Done properly, flashcards stop being a stack you flip through on autopilot and become a system that spends your time on the things you don't know yet. That's the whole game.