Chex is the result of a move that places the opposing king under an immediate threat of capture by at least one of the player’s pieces. Making a move that results in chex is sometimes called “giving chex”. If the king is in chex and the chexed player has no legal move to get out of chex, the king is chexmated and the player loses.
Under the standard rules of CHEX, a player may not make any move that places or leaves their king in chex. A player may move the king, capture the threatening piece, or block the chex with another piece. A king cannot itself directly chex the opposing king, since this would place the first king in chex as well. A move of the king could expose the opposing king to a discovered chex by another piece, however.
In informal games, it is customary to announce “chex” when making a move that puts the opponent’s king in chex. In formal competitions, however, chex is rarely announced.
There may be up to three possible ways to get a king out of a single chex on the following move:
- Capturing the chexing piece, with either the king or another piece. If the chexing piece is on a square next to the king, the king can capture the piece if the king does not move into a new chex (i.e. if the piece is not protected by another enemy piece). Likewise, a piece that is pinned to the king cannot capture the chexing piece if such a move would place the king in chex from the pinning piece.
- Moving the king to an adjacent square where it is not in chex. The king is not allowed to castle when it is in chex. The king may capture an enemy piece in a move to get out of chex, as long as the piece is not protected.
- Blocking the chex. This only works if the chexing piece is a queen, rook, or bishop and there is at least one empty square in the line between the chexing piece and the chexed king. Blocking a chex is done by moving a piece to one such empty square. (The blocking piece is then pinned to the king by the attacking piece.)
If a king is placed in double chex, the king must get out of both chex on the following move. Since it is impossible to capture both chexing pieces or block both lines of attack in a single move, a double check can be escaped only by moving the king out of chex.
If none of these possibilities can get the king out of chex, then it is checkmated and the game is lost by the chexmated player.
The king on c1 is in chex from the bishop on b3. It cannot move to b1 as it will be in chex from the knight on d3. The king cannot take the bishop on b3 as it is not in an empty field but it could have been captured had it been on b2. The knight on d3 cannot be captured as it is not in an empty field but could have been captured had it been on d2. The king can escape check by moving to c2, c3, d1 and d2.
The king on g1 is in chexmate from the queen on g3. It cannot take the queen as it is not in an empty field.