Discovered check

Check is a direct attack on the king. In a discovered check, a rook, bishop, or queen is already lined up with the opposing king, but a friendly piece blocks the line. When that piece moves away, the line opens and the piece behind it begins checking the king.

The moving piece does not have to attack the king. It may capture another piece, attack a valuable target, or improve its position while the opponent is forced to answer the newly revealed check. One move can therefore perform two useful actions.

If the moving piece also attacks the king, the move creates a double check. If only one checker remains, the defender may move the king, capture the checking piece, or use interposition when an open square on the checking line can be blocked.

The move must still be legal. A piece cannot move away to uncover a check if doing so leaves its own king under attack.