Chess

About this game

This is complete chess, not a simplified version. Castling on both sides is offered only when the king and rook have not moved, the squares between them are empty, and the king is neither in check nor passing through or landing on an attacked square. En passant is available for exactly one move after the enemy pawn's double step. Pawns reaching the last rank open a promotion chooser — queen, rook, bishop, or knight. Illegal moves are never offered in the first place: every candidate move is played out internally and discarded if it would leave your own king in check, so a pinned piece simply has no squares along which it could expose the king.

The game ends by checkmate or stalemate, and also declares the three standard draws automatically: threefold repetition of the same position, the fifty-move rule (fifty moves by each side with no capture and no pawn move), and insufficient material — king versus king, king and a single minor piece, or king and bishop versus king and bishop on the same colour squares.

The computer opponent is a real chess engine, and it thinks in the background, so the page stays responsive however long it takes. It searches ahead with alpha-beta and plays out capture sequences before judging a position, remembers positions it has already worked out, looks at the most promising moves first and spends little time on hopeless ones. It judges positions on material, piece placement (blended from opening to endgame, so its king hides early and becomes active late), pawn structure (doubled, isolated and passed pawns), rooks on open files, the bishop pair, how freely its pieces move and the pawns in front of its king. From level 3 it opens with a short book of main lines, so games start sensibly and differently.

There are ten levels. Level 1 often plays a random move and is beatable by a newcomer; levels 2 to 6 look further ahead and make fewer mistakes, each choosing among its better moves with a little less randomness; from level 7 it always plays the best move it finds, thinking for up to about 1, 2, 4 and 8 seconds at levels 7 to 10. In testing, levels 3, 5 and 7 each won every game against the level two below. Hint shows the move the computer would play for you (at level 8).

Puzzles. Checkmate puzzles from mate in 1 up to mate in 3, each one proven by a solver to be a forced mate with a single winning first move. Any move that still forces mate in time is accepted, not just the stored line, and the opponent always answers with its most stubborn defence. Hint highlights the piece to move. Solved puzzles are remembered on this device, and puzzles never touch your saved game or your win/draw score. There's also a daily puzzle, the same for everyone, on the Games page.

Undo steps back one full move against the computer (taking back its reply and your move, so it stays your turn) and one half-move in pass-and-play. Board flip, last-move highlighting, legal-move dots, and a live algebraic move list are all built in. Your game in progress — position, turn, castling rights, en passant target, move history, and mode — is saved on this device, so closing the tab and coming back picks up exactly where you left off.