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What is spaced repetition?

Last updated July 7, 2026 · 5 min read

How spaced repetition works

The mechanics are simple even though the underlying memory science isn't. You learn something new — a word, a fact, a fact you keep mixing up. Instead of reviewing it again tomorrow and the day after and the day after that, you review it once, wait a bit, and review it again. If you got it right, the wait before the next review gets longer. If you got it wrong, the wait shrinks and you see it again sooner. Repeat that enough times and the gaps stretch from a day to a few days to weeks to months, each successful review buying you a longer break before the next one is needed.

The word "spaced" is doing real work here — it's specifically about the gaps, not just about reviewing more than once. Two reviews five minutes apart barely count; two reviews properly spaced days apart do most of the work.

Why timing the review matters

Cramming — reviewing something over and over in one sitting — feels productive because the material looks familiar by the fifth pass. But that familiarity fades fast once the session ends, because nothing forced your memory to actually hold on between reviews. Spaced repetition works by the opposite logic: it deliberately lets a little forgetting happen between reviews, because the mild struggle to recall something you're about to lose is what strengthens the memory trace. Review too early, before any forgetting has started, and you waste the session reinforcing something you didn't need reinforced. Review too late, after you've already forgotten it, and you're relearning from scratch instead of reinforcing. The useful window sits in between — timed to just before you'd have forgotten.

What is FSRS?

FSRS (Free Spaced Repetition Scheduler) is a modern algorithm for calculating that timing automatically, item by item. Older systems, like the classic Leitner box, use one fixed rule for everyone: get a card right, it moves up a box reviewed less often; get it wrong, it drops back down. FSRS instead builds a small memory model from how you've actually performed on that specific item — how many times you've seen it, how you did each time, how long it's been — and predicts when you personally are likely to forget it. CatchFrase's smart review runs on FSRS, so the timing on "cup" and the timing on a trickier word like "notebook" aren't on the same fixed clock; each one is scheduled based on how solid it actually is for you.

Spaced repetition vs. cramming

Cramming optimizes for tomorrow's quiz. Spaced repetition optimizes for actually knowing the material weeks or months later, at the cost of feeling less immediately "done." If you only need a word for one conversation tomorrow, cramming might get you there. If you want it to still be there when you need it in six months — which is the entire point of building a vocabulary — spaced review is the mechanism that gets you there, because it keeps testing the memory right at the edge of forgetting instead of only right after learning.

Common questions

What's the difference between spaced repetition and flashcards?

Flashcards are the format — a question on one side, an answer on the other. Spaced repetition is the schedule that decides when you see each card again. You can use flashcards without spaced repetition (reviewing the whole deck daily) or spaced repetition without physical flashcards (any format where you retrieve an answer and get timed feedback works).

How is FSRS different from older spaced repetition systems?

Older systems like the Leitner box use fixed rules — get a card right, it moves to a box reviewed less often, on a preset schedule that's the same for everyone. FSRS instead builds a memory model from your actual performance on that specific item and predicts when you personally are likely to forget it, adjusting the next review time individually instead of applying one rule to every card.

Does CatchFrase use spaced repetition?

Yes. CatchFrase's smart review runs on FSRS, timing each word's next review to just before you're likely to forget it, based on how you've done with that word before.