What is SM-2, where did it come from, and where is it used?

SM-2 (SuperMemo-2) is Piotr Woźniak's 1987/1990 spaced-repetition algorithm — for decades the default scheduler in Anki and the basis for Quizlet, Memrise and others.

Per card, SM-2 stores just three things:

• an ease factor (EF) — a per-card multiplier for how fast its interval grows,
• a repetition count — how many times in a row you've recalled it, and
• the last interval — how many days since the previous review.

After each review you rate your recall, and SM-2 uses that rating to update the interval and the ease factor. It's simple enough to run on paper, which is exactly why it spread so widely.