A pin is a tactic where a piece cannot move without exposing a more valuable piece behind it. An absolute pin is against the king — the pinned piece is legally frozen, because moving it would leave the king in check. A relative pin is against a valuable piece like the queen, where moving is legal but usually loses material. Bishops, rooks, and queens create pins along diagonals, ranks, and files. A pinned piece is a weak defender, so a common plan is to pile more attackers onto it and win it, since it cannot run away.