You've probably heard that "spaced repetition" is the secret to remembering things long term. What you may not have seen is how unglamorous the actual mechanism is. Behind Semantix's flashcards is SM-2 - an algorithm from the late 1980s that's still the backbone of modern review apps. Once you see how it works, you'll know exactly why your reviews are scheduled the way they are.
The problem it solves: the forgetting curve
Learn a new word today and your memory of it decays - fast at first, then more slowly. Review it just before you'd forget, and the curve resets, but flatter: the next time, it takes much longer to fade. Review it again at the right moment and it flattens further still.
The trick is timing. Review too soon and you waste effort on something you already know. Review too late and you've forgotten it and have to relearn from scratch. Spaced repetition is just the discipline of reviewing each item at the last useful moment - and SM-2 is one way to estimate when that moment is.
SM-2 in plain terms
Every card you study carries three numbers:
- Ease factor - how "easy" this card is for you, starting at 2.5. A
higher ease means longer gaps between reviews.
- Interval - how many days until the card is due again.
- Repetitions - how many times in a row you've recalled it successfully.
When a card comes up, you grade how it went - roughly, from "total blank" to "instant and effortless." SM-2 turns that grade into the next interval.
If you recalled it
- First successful review: see it again in 1 day.
- Second: in 6 days.
- After that: the new interval is the previous interval times your current
ease factor. So a 6-day card at ease 2.5 jumps to about 15 days, then ~38 days, then a couple of months, and so on - each gap roughly 2.5x the last.
The better you remember a card, the faster those intervals balloon, until you're reviewing a well-known word only once every few months.
If you forgot it
A lapse is a reset. The repetition count drops back to zero and the interval returns to 1 day - you'll see the card tomorrow and have to rebuild the streak. The card isn't gone, it's just back near the start of the ladder.
The ease factor drifts to fit you
After each review, SM-2 nudges the ease factor based on how hard the recall felt. Cards you find easy drift toward longer and longer gaps; cards you keep fumbling get a lower ease and stay closer together. Over time the schedule adapts to each card and each learner - your "hard" words and your "easy" words get genuinely different treatment, automatically. (SM-2 keeps the ease factor from dropping below 1.3, so even a stubborn card never collapses to daily reviews forever.)
Why this fits Semantix so well
Here's the elegant part: SM-2 needs exactly one input from you - how the review felt - and it handles the scheduling math itself. That maps perfectly onto how Semantix builds your deck.
You have a conversation. The tutor turns the words you used into cards. SM-2 then schedules each one based on your performance, so the words you struggle with surface often and the ones you've nailed quietly fade into the background until they actually need a refresh. You're never manually deciding what to review or when - the algorithm spends your study time where it earns the most retention.
The takeaway
Spaced repetition feels almost too simple once it's demystified: track an ease factor, grow the interval when you succeed, reset it when you fail, and let the gaps stretch out as memories strengthen. SM-2 has powered serious learners for over thirty years, and it's quietly working every time a Semantix card comes due at just the right moment.
Want to see it in action? Start learning and your first review will be scheduled before you finish your first chat.