The Leitner system, explained
The Leitner system is a spaced-repetition scheduler you could run with five shoeboxes and a stack of cards. It is simple enough to explain in a paragraph and good enough that software still uses it fifty years later.
The rule
Every word lives in one of five boxes. A new word starts in box 1. Answer it correctly and it moves up one box. Answer it wrongly and it goes straight back to box 1, no matter how high it had climbed.
Each box has a longer review interval than the one below it. Box 1 comes back almost immediately; box 5 might not come back for weeks. So a word you keep getting right rapidly stops taking up your time, and a word you keep failing keeps reappearing until it sticks.
Why the reset is so aggressive
Dropping a word all the way to box 1 on a single mistake looks harsh, and it is the part people most often want to soften. It is worth keeping.
A word in box 4 that you just failed was, by definition, scheduled wrongly — the system believed you knew it and you did not. The cheapest way to recover from a bad estimate is to throw it away and re-measure. Softening the reset mostly produces a queue full of words you believe you know.
What "mastered" means
In LinguaSwap a word that reaches box 5 counts as mastered, and the statistics page reports how many of your words have got there. Box position is shown as a red-to-green ramp, so a library that is mostly green is one you have genuinely learned rather than one you have merely seen.
Mastery is per direction. A word can sit in box 5 for Spanish to English and box 1 for English to Spanish, because producing a word is a harder skill than recognising it. Treating those as one number would flatter you.
When to use something other than the schedule
The schedule optimises for long-term retention, which is the wrong objective the night before an exam. That is what the other practice modes are for: Cram runs the whole library ignoring the schedule, and deliberately does not record box changes, so a panic session does not corrupt weeks of scheduling data.
Similarly, Weak Words pulls the lowest boxes and most-missed items first when you want to attack trouble spots directly, and Journey walks a large library end to end, unlocking new words only as the current set is mastered.
Common questions
How many Leitner boxes should there be?
Five is the common choice and what LinguaSwap uses. More boxes give finer-grained intervals but take longer to move a word through; fewer make the jumps too coarse.
Why does a wrong answer send the card back to box 1?
Because the failure is evidence that the current interval was too long. Resetting is the cheapest way to re-measure how well the word is actually known.
Does the Leitner system handle typos?
LinguaSwap grades answers after trimming whitespace and normalising Unicode, and is case-insensitive except where capitalisation is grammatical, as in German. Accents are always significant, because an accent is part of the word.
The Leitner system, explained
The Leitner system is a spaced-repetition scheduler you could run with five shoeboxes and a stack of cards. It is simple enough to explain in a paragraph and good enough that software still uses it fifty years later.