A pin is a tactical device in which an enemy piece cannot (or should not) leave the line of attack because the king or a more valuable piece stands behind it. Only long-range pieces can pin: the bishop, the rook and the queen.
Absolute pin
If the king stands behind the pinned piece, moving it off the line is illegal — it would expose your own king to check.
1. 2. 3.
As long as the pin is not broken, the white knight on c3 cannot move, and so it does not control e4 and d5.
Relative pin
If the piece behind the pinned one is not the king but, say, the queen or a rook, moving off the line is legal but usually unfavourable: a more valuable piece is lost.
Such pins sometimes turn out to be false. A famous example is Légal's mate: Black “pinned” the knight to the queen, but the knight moved anyway and the pin turned into checkmate.
How to exploit a pin
- Attack the pinned piece again: it cannot move and is hard to defend.
- Attack it with pawns: a pawn is cheaper and wins material.
- Remember that a pinned piece defends badly: it cannot leave its line to cover another square.
How to get out of a pin
- Put another piece between the pinned piece and the king (queen).
- Move the king or queen off the line.
- Attack the pinning piece — for example, play a pawn to a3, h3 or h6 against a bishop.
A related device, where the more valuable piece is attacked first and the less valuable one behind it afterwards, is called a skewer. Attacks “through” a piece are covered in the article “X-ray”.