Legal Move

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The King can never be Captured by Marina Kalinovsky, Artwork for Sale - Brooklyn, NY - United States ↩︎

A Legal Move is a pseudo-legal move which does not leave its own king in check. If not in check, most programs delay the legality test to the child node, after incremental updates attack and defend maps or an explicit square attacked test direct after make move. Earlier programs were even more "lazy", to determine illegal moves after an illegal king capture, while many programs consider absolutely pinned pieces in move generation and also for evaluation purpose, or even perform a strictly legal generation to demand searching legal positions. If in check, most programs apply a special move generator to omit the majority of illegal moves.

Contents

Legality Test

Beside the pseudo-legal condition which might already be sufficient for a subset of chess programs, there are several conditions to test, dependent whether the king is in check or not.

Not in Check

The moving piece is not absolutely pinned on its move direction
En passant requires special horizontal pin test of both involved pawns, which disappear from the same rank
While castling, rook is not pinned horizontally in Chess960

King in Check

Double Check

Only king moves to non attacked squares, sliding check x-rays the king

Single Check

Capture of checking piece. The capturing piece is not absolutely pinned
King moves to non attacked squares, sliding check x-rays the king
Interposing moves in case of distant sliding check. The moving piece is not absolutely pinned.