In chess, a pin is a situation in which a piece is forced to stay put because moving it would expose a more valuable piece behind it to capture. In effect, pinned pieces are blocking a check on a king or blocking an attack on a more valuable piece.
In the diagram to the right, the black knight is pinned to the black king by the white bishop. This is an absolute pin, because the rules forbid moving the knight, as it would expose the king to attack.
The black rook is pinning the white knight to the white queen. This is a relative pin; White is unlikely to move the knight because this would lose the queen, a far more valuable piece—but White still has the choice.
Only pieces that can move an indefinite number of squares in a horizontal, vertical, or diagonal line can pin opposing pieces down. Because bishops can move diagonally, rooks can move horizontally or vertically, and queens can move in all three of these directions in a line, these three kinds of pieces can pin opposing pieces down, relatively or absolutely. Kings, knights, and pawns cannot pin other pieces down. Any piece can be pinned down except the king, because the king is the most valuable piece a player has.
Since the black queen is pinned to the black king by the white rook, the queen cannot be moved off the e-file. This is an example of a partial pin.
In cases of a full pin, the pinned piece cannot move at all without exposing its valued piece to attack. In cases of a partial pin, the pinned piece can still move along the line of linear attack (such as along a file, rank, or diagonal), but were it to move off this line of attack, the valued piece would be exposed to the attack. In many cases, such a partially pinned piece may be able to capture the pinning piece to end that pin.
It is possible for two opposing pieces to be partially pinning each other. It is also possible for one piece to be absolutely pinned in one direction and/or relatively pinned in one or more other directions (orientations).
The act of breaking a pin is unpinning. This can be executed in a number of ways: the piece effecting the pin can be captured; a piece can be put between the pinning unit and the pinned unit; a piece can be put between the pinned unit and the unit to which it is pinned; or the unit to which a piece is pinned can be moved.
Although the chess tactics article does not specifically categorize pins as tactics, they are useful in tactical situations. One tactic which takes advantage of a pin can be called working the pin. In this tactic, other pieces from the pinning piece's side attack the opposing pinned piece. Since the pinned piece cannot move out of the line of attack, the pinned piece's player may move other pieces to defend the pinned piece, but the pinning player may yet attack with even more pieces, etc. Pinning can also be used in combination with other tactics. For example, a piece can be pinned to prevent it from moving to attack, or a defending piece can be pinned as part of tactic undermining an opponent's defense. A pinned piece can no longer be counted on as a defender of another friendly piece (that is out of the pinning line of attack) or as an attacker of an opposing piece (out of the pinning line). However, a pinned piece can still check the opposing king.
A pin that often occurs in openings is the move Bb5 (see algebraic chess notation) which, if Black has moved their d-pawn, pins the knight on c6, because moving the knight would expose the king on e8 to check. A common way to win the queen is to pin her to the king with a rook: for instance with a white rook on e1, the black queen on e5 and the black king on e8.
Sometimes in a chess game position, a piece my be considered to be in a situational pin. In a situational pin, moving the pinned piece out of the line of attack will result in a situation detrimental to the player of the pinned piece, such as a checkmate. Although a situational pin is not an absolute pin and the pinned piece can still be moved according to the rules, moving out of line of attack can result in a bad situation or even immediate loss of the game.
White to move. Should White's knight capture the black bishop?
Consider the chess position shown at right. White has not castled or moved the king or rook yet. The black bishop has just moved from e6 to d5, making itself unprotected and available for capture by the white knight on b4. It is now white's turn to move. Should white's knight capture the black bishop?
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