Illegal move

A move is illegal when it fails any rule that applies to the position. The problem may be the piece's movement, a blocked path, an unmet special condition, or the fact that the player's own king remains under attack.

Common cases

• Moving a rook diagonally or a pawn backward.
• Making a bishop, rook, or queen pass through another piece.
• Moving the king onto an attacked square or failing to answer an existing check.
• Attempting castling after the king has moved, through an attacked square, or with pieces in between.
• Trying an en passant capture after its one-move opportunity has expired.

Incorrect geometryBlocked pathSelf-check or unresolved checkMissing special condition

A rook does not move diagonally

8

7

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5

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2

a1

b

c

d

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f

g

h

d5 is on the same file as the rook on d4, and the path is clear.

The attempted Re5

White to move. The rook on d4 can move to d5 because the square lies on the same file and the path is clear. Re5 is illegal by geometry because e5 is diagonal from d4.

1. d5 is on the same file as the rook on d4, and the path is clear.

In a competition played in person with physical pieces, the arbiter, the official who applies the rules, follows the correction and penalty procedure set for that event and speed of play. On a digital platform, the interface usually rejects the attempt before it becomes a played move. A poor move that still follows every rule is not illegal; it is simply a mistake.