How it works
The puzzles are 44,943 positions from real games, taken from the Lichess puzzle database (public domain). Only well-tested puzzles are included (popular, played at least 300 times, with a settled rating), balanced across every rating from 400 to 3000 and covering all 73 Lichess themes. Every puzzle was replayed through this site's rules engine when the set was built.
Solving. The opponent's move is played first; then find the best reply. Puzzles can be several moves long: the opponent answers each correct move. As on Lichess, any move that gives checkmate counts too. Click a piece and a square, or type the move.
Your rating uses Glicko-2, the system Lichess uses. Each puzzle counts as a game against the puzzle's rating: solving it cleanly is a win; a wrong move, a hint or the solution is a loss. The number after ± is the uncertainty; it shrinks as you solve more. Rated and theme puzzles are chosen near your rating, a little above on average. Everything is kept in this browser only.
Review is spaced repetition. A rated puzzle you miss comes back the next day, then after 3, 7, 16 and 35 days each time you solve it, until it's learned; a miss starts it over. Mistakes from your reviewed games against Stockfish join in as puzzles of their own: the opponent's move, then find what you should have played. Since only the engine's move is stored, "Mine was fine" lets you count another move you're sure of. Everything feeds My training.
Themes come from the course: each tactics and endgame lesson links here with puzzles of that theme at your level. Daily puzzle is the same puzzle for everyone on a given day. Timed run gives you three minutes: puzzles start easy and get harder with each one solved, and a wrong move costs ten seconds and moves on to the next puzzle.