alt Hacker NewsWell, if a piece is pinned it's illegal to move it.
Rule 3.9.2: No piece can be moved that will either expose the king of the same colour to check or leave that king in check.
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You can also pin a pawn to a queen, but the pawn can still legally move.
The point is that, logically, the first part of that rule (“expose the king”) is implied by the second part (“leave that king”), so the first part is redundant. You could simplify the rule to:
No piece can be moved that will leave the king of the same color in check.
Pinning isn’t a rule, it’s just something that arises from other rules.
Also, pinning can happen with pieces that don’t include a king, which means you can just move out of the pin and expose whatever other piece.
It’s just a chess tactic, not a rule. It’s like saying a chess skewer is a rule too.
Unlike en-passant and castling, pinning and discovered checks are consequences of lower-level rules.
At the "Is this move legal?" level, they don't need unique rules of its own if the lower-level rules are specified correctly.