Tactical motifs · Advanced
pin
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The idea
A piece stands in front of a more valuable one and cannot move: the moment it steps aside, the attack runs through and takes what stood behind it.
A pinned piece is effectively glued in place. It stays on the board but stops working — it can no longer defend anything or attack anything.
How it looks
A bishop strikes along the diagonal: first the enemy queen, then the king behind her. The queen cannot leave the diagonal — the king is behind her, and stepping into check is illegal. This is an absolute pin: moving away is forbidden by the rules. The queen may only move along that same diagonal, but the bishop is protected, so at best the queen is traded for the bishop.
There is a softer version: the pinned piece stands not in front of the king but in front of a queen or rook. It may move, but then whatever stands behind it is lost. That is a relative pin — the piece is not forced to stay, but moving costs too much.
How to spot it
Look along the lines and diagonals leading to your king. If there is nothing between your piece and your king, and a rook, bishop or queen sits on the far end, your piece is pinned.
A pinned piece is often attacked again: it cannot run, so you simply pile on more attackers.
Often confused with
A skewer. The geometry is identical — two pieces on one line. The difference is who stands behind: in a pin the piece behind is more valuable, in a skewer it is less valuable.