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In chess, a pin is a situation brought on by an attacking piece in which a defending piece cannot move without exposing a more valuable defending piece on its other side to capture by the attacking piece. "To pin" refers to the action of the attacking piece inducing the pin, and the defending piece so restricted is described as pinned.
Only pieces that can move an indefinite number of squares in a horizontal, vertical, or diagonal line, i.e., bishops, rooks and queens, can pin opposing pieces. Kings, knights, and pawns cannot pin. Any piece may be pinned except the king, as the king must be immediately removed from check under all circumstances.
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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. A relative pin is one where the piece shielded by the pinned piece is a piece other than the king, but typically 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 at right.)
If a rook or queen is pinned along a file or rank, or a bishop or queen is pinned along a diagonal, the pin is a partial pin: the pinned unit can still move along its line but cannot leave that line. A partially pinned unit may break its own pin by capturing the pinning piece; however, a partial pin can still be advantageous to the pinning player, for instance if the queen is pinned by a rook or bishop, and the pinning piece is defended, so that capturing it with the queen would lose material. Note that a queen can only ever be partially pinned, as it can move in any linear direction.
It is possible for two opposing pieces to be partially pinning each other. It is also possible for one piece to be pinned in one direction (line of attack) and partially pinned in another, or otherwise pinned in two or more directions.
The act of breaking a pin is unpinning. This can be executed in a number of ways: the piece creating the pin can be captured; another unit can be moved onto the line of the pin; or the unit to which a piece is pinned can be moved.
Although a pin is not a tactic in itself, it can be 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 usually 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 - and therefore still can defend friendly pieces against captures made by the enemy king.
A pin that often occurs in openings is the move Bb5 which, if Black has moved ...Nc6 and ...d6 or ...d5, pins the knight on c6, because moving the knight would expose the king on e8 to check. (The same may, of course, occur on the other flank, with a bishop on g5, or by Black on White, with a bishop on b4 or g4.) A common way to win the queen is to pin her to the king with a rook: for instance with the black queen on e5 and the black king on e8 and no other pieces on the e-file, the move Re1 by White would pin Black's queen.
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The white pawn on g2 cannot take the deadly black rook on h3 because that pawn is pinned by the black queen.
In the diagram at right with white to move next, Black is threatening the following rook sacrifice leading to mate.
The pawn on g2 cannot take the rook on h3 because the queen on g3 is pinning the pawn with a vertical line of attack. The only move to prevent the above moves is 27.Nf4 which temporarily blocks black's bishop from protecting his queen, but to no avail. The black bishop can take the knight by 27...Bxf4 renewing the same threat of mate in 2, or Black can respond as follows to mate anyway:
In this case, white could not take the mating rook now on f3 with the g2 pawn because the queen on h2 would now be pinning the pawn with a horizontal line of attack. With mate against him being inevitable, white resigned after move 26.
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Sometimes in a chess game position, a piece may 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.
Consider the chess position shown at right. 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. White should not capture the black bishop or otherwise move the knight because after 1.Nxd5, 1...Rb1+ wins white's rook, because the king is forced to move away from the check, thereby exposing the rook to attack (a skewer).