A card’s retrievability is the probability you’ll be able to successfully recall the answer within a short amount of time, if the card is presented to you for review. Retrievability decreases steadily over time, then jumps up to near 100% again when you recall the answer.
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In spaced repetition practice, we aim to always keep the retrievability of our cards higher than some threshold (often called desired retention or forgetting index), so that we always have a good chance of remembering them when we need them. The amount of practice required increases exponentially as the desired retention approaches 100% – that is, each additional minute we spend practicing boosts our memory less than the minute before did – so most spaced repetition systems aim to maintain 85–90% retrievability, which is a good balance between usefulness and practice time. When a card's estimated retrievability drops below the desired retention threshold, it enters your queue for practice.
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An important but poorly understood point is that your retrievability when recalling a random card at a random time – like when you need it in real life or on an exam – is significantly higher than the desired retention, as long as you stay more or less up to date on practicing your cards. That’s because the moment the scheduler puts a card in your practice queue is the moment you know it worst: your memory is now bad enough that you need a refresher, and once you get it, your memory will be much better again. A randomly selected card at a random time will, on average, be some time away from its next review, and thus have a higher retrievability. The math of actual moment-in-time retrievability is too complex for this article, but a good rule of thumb is that if your target retrievability is 90%, your actual retrievability on a random card at a random time is 95%.
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A card’s stability is how firmly the idea is stored in your memory. The higher a card’s stability, the slower its retrievability decreases over time. The more times you successfully recall a card, the higher its stability. Stability corresponds to a card’s interval parameter in a spaced repetition system, i.e., the number of days before the retrievability drops below some desired value.

Retrievability Period (temporary increase in retrievability): On your exam date, you want your retrievability to be as high as possible. As discussed earlier, it’s impractical to maintain a retrievability of, say, 98%, constantly. But doing extra practice to reach a very high retrievability for the day of your exam is easy enough, if you’ve already achieved a solid understanding of those cards with a high stability. To make this happen, we add extra practice for all cards just before your exam.
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Maintenance Period (temporary decrease in retrievability): If you have an exam far in the future and you’re learning material you won’t need to know until the exam, keeping retrievability high in the meantime might be a poor use of your time. If you choose a “reduced” burden during the maintenance period, we’ll decrease the desired retention to 70%. As your exam date approaches, we’ll add a Consolidation Period where the desired retention returns to normal, adding extra practice as needed.
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(Why 70%? Reducing retention below 60–70% results in increased practice time, because the time spent relearning cards you’ve forgotten begins to exceed the time saved by practicing less often.)
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Extra Practice Sessions: If you’re struggling to learn a card or you’ve recently forgotten it, and your exam is coming up in a day or two, you need to relearn it now, not at a “more efficient” time that comes after your exam. RemNote will compress the practice schedules of forgotten cards near your exam date so that you relearn them before the exam.