The Leitner system: how five boxes help you remember

The Leitner system is a way of studying flashcards with spaced repetition: cards you know are reviewed less and less often, cards you forget come back soon. New cards start in Box 1. A card you answer correctly moves up one box and waits longer before its next review; a card you get wrong goes back to Box 1. In the Flashcards app the five boxes are reviewed after 1, 3, 7, 14 and 30 days.

The result: you spend your time on the cards you are about to forget, not on the ones you already know.

The method is named after the German science journalist Sebastian Leitner, who described it in his 1972 book So lernt man lernen (“How to learn to learn”). His version used a paper card box with compartments of growing size. A card moved one compartment further each time you knew it and went back to the first compartment when you did not. You reviewed a compartment when it filled up, so cards you knew well were seen less and less often.

Apps keep the same idea but replace “when the compartment is full” with a fixed review date for every box.

Rate a card Again and it returns to Box 1, no matter which box it was in. In Flashcards it also comes back once more at the end of the same session, so you see the answer again while it is fresh, and it is due again tomorrow. Even if you get that retry right, the card stays in Box 1 for the day: one miss in a session is enough to restart the climb.

For cards that are not wrong but shaky, there is a third option: mark the card as hard. Hard cards are due every day and appear twice in each session, so you practise them more often without losing their box.

Two well-researched effects explain why the boxes work. The spacing effect: reviews spread over days and weeks lead to better long-term memory than the same number of reviews crammed into one sitting. A meta-analysis by Cepeda and colleagues (2006) pooled 317 experiments and found this benefit consistently. The testing effect, also called retrieval practice: actively recalling an answer strengthens memory more than rereading it. In a study by Karpicke and Roediger (2008), students who kept testing themselves on word pairs remembered about 80 % a week later; students who stopped testing once they had recalled a pair remembered only about a third.

Both ideas go back to Hermann Ebbinghaus, who measured in 1885 how quickly he forgot lists of syllables. His forgetting curve drops steeply in the first days and then flattens, and every review slows the forgetting down further. That is why the gaps between the Leitner boxes grow from 1 to 30 days, and why many learners in Japan and China know the same idea simply as reviewing along the Ebbinghaus curve.

Apps such as Anki calculate a separate interval for every card. SM-2, the SuperMemo algorithm from 1987, multiplies a card’s interval by an “ease” factor that changes with each answer. FSRS, an open-source scheduler available in Anki, estimates how likely you are to still remember a card and schedules the review just before that probability drops below your target.

These algorithms can save some reviews, but the schedule is hard to see and depends on settings. The Leitner system trades that fine-tuning for predictability: five boxes, fixed intervals, and you always know where a card stands and when it comes back. If you are weighing the two approaches, see our honest comparison with Anki and with Quizlet.

A paper card box works and costs nothing: you need a box with five compartments, cards, and the discipline to check each compartment on time. An app takes over the bookkeeping. It stores a due date for every card, shows only what is due today, reminds you, and never loses a card. It also lets you add photos and study on the bus. The Flashcards app follows exactly the five-box rules described here, so you can switch between paper and app without relearning anything. It is especially handy for vocabulary and exam preparation.