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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.
Example One 
Suppose you have 3 boxes of cards called Box 1, Box 2 and Box 3. The cards in Box 1 are the ones that you often make mistakes with, and Box 3 contains the cards that you know very well. You might choose to study the Box 1 cards once a day, Box 2 every 3 days, and the Box 3 cards every 5 days. If you look at a card in Box 1 and get the correct answer, you "promote" it to Box 2. A correct answer with a card in Box 2 "promotes" that card to Box 3. If you make a mistake with a card in Box 2 or Box 3, it gets "demoted" to the first box, 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.
Example Two 
This example uses 5 proficiency levels and 12 decks of flash cards. Cards at Proficiency Level 1 are reviewed at every learning session; those at Level 5 are retired and no longer in use. Those at Levels 2, 3, and 4 are reviewed every 2nd, 3rd, and 4th session, respectively.
Learning sessions are numbered from 0 to 9, then the numbering starts over again (that is, 0, 1, 2, ... 8, 9, 0, 1, 2 ...). Cards at Level 1 are in Deck Current; those at Level 5 are in Deck Retired; all other cards are in 1 of these 10 "progress" decks, each of which begins with a title card sporting 4 digits:
0-2-5-9 • 1-3-6-0 • 2-4-7-1 • 3-5-8-2 • 4-6-9-3 • 5-7-0-4 • 6-8-1-5 • 7-9-2-6 • 8-0-3-7 • 9-1-4-8
If a learner is successful at a card from Deck Current, it gets transferred into the progress deck that begins with that session's number. (For example, success at a card during Session 6 transfers it from Deck Current to Deck 6-8-1-5.) Cards from that deck are reviewed whenever a number from the deck title matches the session number. (For example, cards from Deck 6-8-1-5 will be reviewed again at Sessions 8, 1, and 5.) If a learner has difficulty with a card during a subsequent review, the card is returned to Deck Current; otherwise it stays in its progress deck. When a learner is successful at a card during a session that matches the last number on the deck (for example, Session 5 for Deck 6-8-1-5), that card goes into Deck Retired, and the title card for that progress deck is freed up for use at the following session.