📏 interval I — days from the last review to the due date, chosen so that predicted recall at the due date equals the 90% retention target. The stages bin this one number: I < 7 d 🌱, 7–21 d 🌿, I ≥ 21 d 🌸 (Anki's 21‑day "mature" convention).

💪 stability S — memory strength, in days: the elapsed time at which predicted recall decays to 90% — so I ≈ S. Each successful review multiplies S by a gain that is larger the closer the review came to forgetting (the spacing effect) and smaller at high difficulty; a lapse (Again) collapses S to a fraction of its value.

🧗 difficulty D — a per‑card parameter on a 1–10 scale. Again/Hard raise it, Easy lowers it, with mean reversion toward the Easy baseline. Stability gains scale with (11 − D), so a D = 10 card's interval barely grows per review while a D = 1 card's grows ~10× faster; D ≥ 8.5 is one arm of the leech 🧛 flag.

🫧 retrievability R — the predicted probability of recall, following a power‑law forgetting curve R(t) = (1 + F·t∕S)^−d: 1 at t = 0, decaying with elapsed time t, more slowly the larger S. The card comes due exactly when R reaches the 0.9 target.