Pin

A pin occurs when a piece stands in front of a more important target on the same rank, file, or diagonal. If the front piece moves away, it opens the attack on what is behind it. Bishops, rooks, and queens create ordinary pins because they attack along lines.

In an absolute pin, the rear target is the king. The front piece cannot make a move that leaves its king in check because that move would be illegal. In a relative pin, moving is legal but commonly allows the loss of a more valuable piece, checkmate, or another serious consequence.

A pin restricts options but does not automatically win material. The defender may capture the pinning piece, interpose another piece, move the rear target, protect it, create a stronger threat, or, in a relative pin, accept the loss in return for something better.

To exploit a pin, first verify that the line will remain open after the intended move. Also ask what the pinned piece was defending. Even with restricted movement, it may still protect squares or move in a way that keeps the king covered.

A skewer looks similar but reverses the order: the more valuable piece is in front, comes under attack, and exposes the rear piece when it moves.