rules

Summary

The rules the engine enforces, and the ways a game ends.

Canonical values

Key	Value	Last changed
CASTLING	both sides. King and rook unmoved, squares between empty, king not in check, not through or onto an attacked square	2026-10-04
EN-PASSANT	yes, on the move straight after the two-square pawn move	2026-10-04
PROMOTION.ENGINE	accepts queen, rook, bishop or knight	2026-10-04
PROMOTION.TODAY	always a queen — nothing asks the player	2026-10-04
PROMOTION.DESIGN	the player chooses, every time	2026-10-04
END.CHECKMATE	yes	2026-10-04
END.STALEMATE	yes — a draw	2026-10-04
END.FIFTY-MOVE	yes — 100 half-moves with no capture and no pawn move	2026-10-04
END.INSUFFICIENT	king against king; king against king and one bishop or one knight; bishops only, all on one square colour	2026-10-12
END.THREEFOLD	yes — a draw when the same position, side to move, castling rights and en passant square occur three times. Declared automatically, no claim	2026-10-12
END.RESIGN	yes — from the pause sheet, behind a confirm dialog	2026-10-12
END.TIME	yes — with a clock on, the player whose time reaches 0:00 loses; a draw if the other side has only a king. Drawn in game/timeout.html	2026-10-12
UNDO	every move can be taken back, to the first. Against the computer, Undo takes back your move and its reply. Undo does not give back clock time or the 15+10 increment: the time was spent	2026-10-12

Rules

• A rule the engine does not have is not shown to the player. How to play lists what is in this table and nothing else.
• A draw names its rule. Today both draw rules end in "Game Over!".
• A game lost on time has its own result. timeout. It was missing until 2026-10-12.

Related

• how-to-play — the rules as the player reads them
• draw — the draw-by-rule result
• stalemate — the stalemate result
• promotion — the picker
• timeout — the out-of-time result

Sources

• src/lib/game.ts, src/store/gameStore.ts