Legal move

A move is legal when the rules allow it in the exact position on the board. It is not enough for the piece to follow its usual movement pattern: blocked paths, the destination square, special rules, and the safety of the player's own king all matter.

Move, path, and destination

Basic move filters

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The rook can travel from a1 to a2: the destination is empty and no piece blocks the path.

The rook on a1

White to move. Ra2 follows the rook's geometry, has a clear path, reaches an empty square, and keeps the king safe. Ra4 is blocked by the pawn on a3, Rc1 ends on the friendly knight, and Rb2 does not match a rook move.

1. The rook can travel from a1 to a2: the destination is empty and no piece blocks the path.

• The piece must move or capture according to its rules, without passing through pieces unless it is a knight.
• The destination square cannot contain a piece of the same color.
• A castling move or an en passant capture must satisfy its extra conditions.
• After the move, the player's own king cannot remain under attack.

Own king safetySpecial condition

The path is not enough

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The rook on e2 interrupts the file between the black rook on e8 and the white king on e1.

A rook acting as a shield

White to move. The rook on e2 blocks the black rook's attack from e8 against the king on e1. Rd2 follows rook geometry, but leaving the e-file would expose the king to check. Re3 keeps the shield in place and passes that test.

1. The rook on e2 interrupts the file between the black rook on e8 and the white king on e1.

When the king is in check, only moves that remove the attack are legal. A legal move can still be a poor decision, lose a piece, or lead to defeat; legal describes compliance with the rules, not strategic quality.

Required response to check

The check must be removed first

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The rook on e8 checks the white king on e1.

The white king is already attacked

White to move, and the rook on e8 checks the king on e1. Kf2 moves the king away from the e-file and removes the attack. Qa4 would attack the rook on e8, but it is illegal because the check against the white king remains.

1. The rook on e8 checks the white king on e1.
2. Kf2 moves the king off the attacked file.