Pseudo-legal move

A pseudo-legal move appears possible when the piece, its path, and the occupied squares are considered. A rook moves along a clear straight line, for example, or a knight reaches one of its normal destinations. Relevant special conditions are also checked, such as en passant, in which a pawn immediately captures an opposing pawn that has just advanced two squares, and castling, the combined king-and-rook move.

The check is not yet complete. The move may expose the moving side's king, leave it in check when the threat had to be answered, or move a piece that was shielding it from an attack. Any such move causes self-check and is not legal.

The term is used mainly in programming. A chess engine, meaning a program that analyzes positions and calculates moves, can generate many pseudo-legal candidates quickly and then reject those that fail the king-safety test. Separating these stages often makes move generation simpler and faster.

The exact boundary can vary between programs, especially for castling and other special moves. The stable idea is that pseudo-legal means a movement-mechanics candidate, not a move already approved by every legality rule.