Illegal Move

Illegal move is a legalistic term imported directly from the vocabulary of law into the codified Laws of Chess, where legality is defined by rule rather than by custom. The FIDE Laws give it a precise circular-free definition at Article 3.10.2: a move is illegal when it fails to meet the relevant requirements of Articles 3.1 to 3.9, the articles that govern how each piece moves. Article 7.5.1 provides that an illegal move is completed once the player has pressed his clock, after which the opponent is entitled to claim it and the position immediately before the irregularity is reinstated. The Laws extend the category beyond piece movement: Article 7.5.2 treats leaving a pawn on the promotion rank as illegal, 7.5.3 covers pressing the clock without making a move, and 7.5.4 covers using two hands for a single move. Penalties are graduated under 7.5.5: the first completed illegal move gives two minutes to the opponent, and the second loses the game, unless the opponent cannot checkmate by any possible series of legal moves, in which case the game is drawn. No source located in this session dates the first appearance of the phrase in the Laws.

First documented: n.a. — n.a. for first appearance of the phrase; current definition is Article 3.10.2 with penalties at 7.5 of the FIDE Laws of Chess in force from 1 January 2023