In chess, a pin is a tactic in which a defending piece cannot move out of an attacking piece's line of attack without exposing a more valuable defending piece. Moving the attacking piece to effect the pin is called pinning; the defending piece restricted by the pin is described as pinned. Only a piece that can move any number of squares along a horizontal, vertical, or diagonal line (i.e. a bishop, rook, or queen) can pin. Any piece can be pinned except the king, which is the most valuable piece. The inverse of a pin is a skewer, in which a more valuable piece under direct attack may move to expose a less valuable piece to an attack. Types Absolute pin An absolute pin is one where the piece shielded by the pinned piece is the king. In this case it is illegal to move the pinned piece out of the line of attack, as that would place one's king in check (see diagram). A piece pinned in this way can still give check or defend another piece from capture by the opposing king. Relative pin A relative pin is one where the piece shielded by the pinned piece is a piece other than the king, but typically is more valuable than the pinned piece. Moving such a pinned piece is legal but may not be prudent, as the shielded piece would then be vulnerable to capture (see diagram). Partial pin In a partial pin, the pinned piece can still move along the line of the pin, and it can capture the piece that is pinning it, but it cannot leave that line. For example, a rook or queen can be partially pinned along a or , or a bishop or queen can be partially pinned along a . Capturing the pinning piece can still be advantageous to the pinning player, as in the example diagram (the pinning rook is defended, so capturing it with the queen would lose ). A queen can only ever be partially pinned, as it can move in any linear direction, while a knight cannot be partially pinned due to its unique movement. The pawn is a more complex case; due to its limited and conditional movement, whether a pin on a pawn is partial depends on the line and direction of the pin and on whether there are opposing pieces available for it to