The core of the method fits in a sentence: review something again just as you are starting to forget it, and each time you succeed, wait longer before the next review.
Everything else is detail about how long to wait.
Why cramming feels better than it is
Read a new word ten times in one evening and it feels firmly learned. The next morning it is fuzzy; a week later it is often gone. Those ten repetitions all happened while the memory was already fresh, so each one added very little.
Spread the same effort out - once today, once in a few days, once next week - and far more of it survives. Psychologists call this the spacing effect, and it is one of the most consistently replicated findings in the study of memory.
Why the gaps widen
After you learn something, memory fades quickly at first and then more slowly. A review resets that fade, and importantly, the decline after a successful review is gentler than the one before.
That is why intervals grow. A typical schedule looks like this:
- Learn it today.
- Check again in about three days.
- Remembered? Next check in a week.
- Remembered again? Two weeks.
- Then a month.
Get one wrong and the gap shrinks, so it comes back sooner. Nothing is lost; the schedule has just learned that this item needs more attention.
A little difficulty is the point
It seems backwards, but a review that takes some effort does more than an easy one. Researchers Elizabeth and Robert Bjork call these "desirable difficulties": conditions that make learning feel slower in the moment but produce stronger memories.
If you review something you can recall instantly, you learn almost nothing from it. If you review it when you have to reach for it, the act of reaching is what strengthens the memory. Widening gaps are a way of arranging for most reviews to land in that productive zone.
Recall, not rereading
Spaced repetition works best when each review is a test rather than a reread. Seeing the word and the definition together feels like practice. Seeing only the word and trying to produce the definition is actual practice.
This is why the answer should be hidden until you have had a go. A widget that shows the term but keeps the meaning back turns every glance at your phone into a small test.
Where the systems came from
In the 1970s Sebastian Leitner described a paper method: flashcards in a row of boxes, where a card you get right moves to a box you review less often, and a card you get wrong goes back to the first. In the 1980s Piotr Woźniak built SuperMemo, and his SM-2 scheduling algorithm became the basis for many of the flashcard apps that followed.
You do not need to understand the algorithms. They are all doing the same thing: stretching the interval after success and shrinking it after failure.
Keeping it small enough to last
The most common way spaced repetition fails is not the method. It is volume. People add hundreds of cards, reviews pile up, a busy week creates a backlog of two hundred, and they quit.
A tiny daily input avoids that entirely. One new item a day, with its later reviews spread across weeks, never produces a backlog you cannot clear in a minute. Over a year it is still more than three hundred things - and, crucially, most of them are still there.
Honest answers matter
The schedule is only as good as your answers. If you say you remembered something you half-guessed, it is pushed further out than you are ready for and you lose it. An honest "no" costs you nothing except seeing it again sooner, which is exactly what should happen.