Absolute pin

An absolute pin occurs when a piece stands between a line attacker and its own king. If that piece leaves the line and exposes the king to attack, the move is illegal. The restriction comes from the rule against leaving the king in check, not from the material values involved.

For example, an enemy rook may attack down a file through a knight with the king behind it. Because none of the knight's jumps can remain on that file, the knight has no legal move that breaks the pin.

Absolute does not always mean completely immobile. A pinned rook, bishop, or queen may sometimes move along the same line without uncovering the king. A piece may also capture the attacker or interpose a defense if the resulting position leaves the king safe.

The pin ends if the king moves, the pinning piece is captured, another piece blocks the line, or the attack's geometry changes. Before treating the pinned piece as lost, all legal defenses should be listed.

In a relative pin, the piece may move because the rear target is not the king. The choice is strategic or tactical rather than a question of legality.