Comment by yewenjie
10 hours ago
> Chess is a lot trickier than it looks. It has so many rules: castling, en passant, pawn promotion, pinning, the discovered check, and the deadlock case of stalemate.
Nit: Pinning and the discovered check are not really rules, but rather names of tactics.
Well, 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.
That rule doesn't mean it's illegal to move a piece that's pinned. It just means that it's illegal to move it to a square that would expose the king. For example a pawn that's pinned vertically can still push forward, it just can't capture diagonally.
That's why treating colloquial concepts like "pinning" as though they are rules in and of themselves is not really precise or productive.
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.
3.9.2: no piece can be moved if that exposes or leaves its own king in check.
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You can also pin a pawn to a queen, but the pawn can still legally move.
You're both right, depending on whether you mean relative pin vs absolute pin.
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.
Pedantically I disagree, to leave something in a condition it must have been in that condition in the first place. We could have a game where you're allowed to place your king in check, but if it is in check at the start of your turn you must fix that.
While we're being pedantic though it's not a property of the piece that might be able to be moved that will place the king in check. It's a property of the move. For example we might imagine you have a rook between an enemy rook and your king. You can move the rook along the line between the enemy rook and the king, but not perpendicular to it.
The rule should be:
No move can be made where the moving players king is in check in the resulting position
You should submit it to FIDE.
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.
And discovered check means that it is not sufficient to check the position of the piece you have moved, you also need to check the position of other pieces to see whether there is a new check.