Check evasion

A king is in check when it is under an immediate attack. A check evasion is any legal move that leaves the king safe. Merely threatening the opponent is not enough: after the move, the player’s own king must no longer be attacked.

The three possible methods

• Move the king to a square that is not an attacked square. The king also may not capture onto a square protected by another enemy piece.
• Capture the checking piece, provided that the capture does not expose the king to another attack.
• Place a piece between the attacker and the king. This works only against a distant rook, bishop, or queen attack and only when at least one square lies between them.

In double check, two pieces attack the king at once. The only possible type of response is a king move, although that king move may capture one of the checking pieces. Blocking or capturing with another piece would remove only one attack.