Self-check

A move causes self-check when, after the move, an opposing piece attacks the moving side's king square. The move is illegal. Kings are not captured in chess; the rules prevent a position in which the opponent could capture the king on the next turn.

Self-check can arise in several ways. The king may try to enter an attacked square, a shielding piece may move and uncover a rook, bishop, or queen line, or a player who is already in check may choose a move that fails to remove every attack on the king.

Capturing the checking piece is also insufficient if another piece still attacks the king's final square. An opposing piece counts as attacking a square even when that piece could not actually move there without exposing its own king. This rule helps explain why the two kings can never stand on adjacent squares.

Many programs first generate a pseudo-legal move, a candidate that follows the piece's movement but has not yet passed the king-safety test. To be fully legal, the side to move must leave its own king outside check.