Pin (chess)

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.

There is an absolute pin on the black knight as moving it would illegally expose the black king to check from the white bishop.
There is a relative pin on the white knight as moving it would allow capture of the white queen by the black rook.

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 it's 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.)

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.

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.

White to move. White's knight should not capture the black bishop. This is an example of a situational pin.

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