The Leitner system
The Leitner system is a manual spaced-repetition method: flashcards live in numbered boxes, and a card you recall correctly graduates to a slower-reviewed box while a card you miss drops back to the first. It is the simple, physical ancestor of every modern spaced-repetition algorithm.
Proposed by German journalist Sebastian Leitner in the 1970s, the system uses a row of boxes reviewed at different frequencies — box 1 every day, box 2 every few days, box 3 weekly, and so on. Get a card right and it moves to the next, slower box; get it wrong and it returns to box 1 to be drilled again.
The genius of the system is that it allocates your attention automatically: hard cards you keep missing stay in the fast boxes and get more practice, while easy cards drift to the slow boxes and stop wasting your time. That is spaced repetition in physical form.
Its limitation is granularity. Boxes are coarse — every card in a box is treated identically — and a missed card loses all its progress. Modern algorithms keep a continuous schedule per card instead of a handful of buckets.
How EverFlip does this
EverFlip is the Leitner system without the shoebox: instead of a few boxes, FSRS tracks a precise, continuous interval for every card — finer-grained scheduling, no physical cards, and nothing to reshuffle by hand.