Attacked square

An attacked square is a square controlled by an opposing piece. The basic test is whether that piece could capture a piece placed there by following its normal movement. The square may be empty. Pawns attack the two forward diagonals even though they move straight; kings attack adjacent squares; and bishops, rooks, and queens attack along lines until the first blocker.

Effect on the king

A king may not move onto an attacked square because it would be in check. Having castling rights is also insufficient if the king is attacked, must cross an attacked square, or would finish on one. Recognizing attacks is therefore essential to every check evasion.

One regulatory detail is especially important: a piece still counts as attacking a square even when it cannot move there because doing so would expose its own king to check. For example, a piece immobilized in front of its king can still prevent the opposing king from entering a square it controls. This definition also prevents the two kings from becoming adjacent.

Attacking a square does not always mean threatening a useful capture on the next move. The regulatory term describes control relevant to king safety, while attack can also mean a broader offensive plan.