The Leitner system is a widely used method to efficiently use flashcards that was proposed by the German science journalist Sebastian Leitner in the 1970s. It is a simple implementation of the principle of spaced repetition, where cards are reviewed at increasing interval.
In this method flashcards are sorted into groups according to how well you know each one in the Leitner’s learning box. This is how it works: you try to recall the solution written on a flashcard. If you succeed, you send the card to the next group. But if you fail, you send it back to the first group. Each succeeding group has a longer period of time before you are required to revisit the cards.
Suppose you have 3 groups called Group 1, Group 2 and Group 3. The cards in Group 1 are the ones that you often make mistakes with, and Group 3 contains the cards that you know very well. You might choose to study the Group 1 cards once a day, Group 2 every 3 days, and the Group 3 cards every 5 days. If you look at a Group 1 card and get the correct answer, you “promote” it to Group 2. A correct answer with a Group 2 card “promotes” that card to Group 3. If you make a mistake with a Group 2 or Group 3 card, it gets “demoted” to the first level, which forces you to study that card more often.
The advantage of this method is that you can focus on the most difficult flashcards, which remain in the first few groups. The result is, ideally, a reduction in the amount of study time needed.