Six difficulty steps build from common sizes to awkward table calculations. Occasional harder examples become more frequent as they go well. Promotion needs at least four comfortable first attempts on harder examples across three different days, with at least 80% comfortable in the last eight. Retries help you learn but do not count towards promotion.

Missed questions return as fresh examples until correct. Repeated struggles bring easier practice. Correct retries and slow answers return tomorrow. Comfortable reviews gradually spread out, up to 60 days; developing skills return within seven days. Earlier levels stay available for review.

Your progress separates first attempts from retries and compares the same skill, difficulty and answer mode. Mental answers are self-marked. Answer times are approximate, omit detected interruptions, and never affect progression. New measurements start with this update; your older history and unlocked levels are preserved.

These are adjustable practice rules, not a validated learning model. Days are counted in UTC. Progress saves to your server after each rating.