A pin happens when a piece is stuck: if it moves, a more valuable piece behind it becomes exposed to attack. Pins are made by long-range pieces: bishops, rooks, and queens.
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There are two kinds of pin:
- Absolute pin: the piece behind is the king. The pinned piece may not leave the line, because that move would be illegal.
- Relative pin: the piece behind is not the king, for example the queen. The pinned piece may move, but it usually costs material.
A pinned piece can still move along the line of the pin, and can even capture the piece pinning it.
