A check is a condition in chess, shogi and xiangqi that occurs when a player's king (or general in xiangqi) is under threat of capture on their opponent's next turn. A king so threatened is said to be in check. A player must get out of check, if possible, by interposing a piece between the threatening piece and the king, capturing the threatening piece, or moving the king to a square where it is no longer in check. If the player cannot move out of check, the game ends in checkmate and the player loses.
A check 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 checks is sometimes called "giving check". If the king is in check and the checked player has no legal move to get his king out of check, the king is checkmated and the player loses.
Under the standard rules of chess, a player may not make any move which places or leaves their king in check. A player may move the king, capture the threatening piece, or block the check with another piece.
A king cannot itself directly check the opposing king, since this would place the first king in check as well. However a move of the king could expose the opposing king to a discovered check by another piece.
In informal games, it is customary to announce "check" when making a move that puts the opponent's king in check. However, in formal competitions, check is rarely announced.
There may be up to three possible ways to get a king out of a single check on the following move:
In the position in the first diagram, White can get out of check by three methods:
If a king is placed in double check, the king must get out of both checks on the following move. Since it is impossible to capture both checking pieces or block both lines of attack in a single move, a double check can be escaped only by moving the king out of check.
If none of these possibilities can get the king out of check, then it is checkmated and the game is lost by the player being checkmated.
A simple and very common type of check is when a piece moves to directly attack the opposing king only by itself. Sometimes such a check is part of a chess tactic such as a fork, a skewer, or a discovered attack on another piece. In some cases, a check can be used to defend against such tactics.
There are also a few more special types of check:
In friendly games, the checking player customarily says "check" when making a checking move. Announcing "check" is not required under the rules of chess and it is usually not done in formal games. Until the early 20th century a player was expected to announce "check", and some sources of rules even required it (Hooper & Whyld 1992:74).
In algebraic chess notation, a checking move is recorded like any other move, except that a "+" is normally written after the move.
In the FIDE rules for rapid chess, if a player leaves their king in check or commits any other illegal move their opponent can claim a win.
Less commonly (and obsolete), the warning garde can be said when a player directly attacks the opponent's queen in a similar way. This was mostly abandoned in the 19th century (Hooper & Whyld 1992:74). The same move can be both check and garde simultaneously. Before the queen acquired its current move (about 1495) the rook was the most powerful piece. At that time the term check-rook was used for a move that checked the king and attacked a rook at the same time (Hooper & Whyld 1992:75).
Sometimes checking an opponent provides no benefit to the checking player. This is called a "useless check" and it may even provide the checked opponent with a tempo (move opportunity) to move the king into a safer position (Hooper & Whyld 1992:437). For example, 1.e4 e6 2.d4 Bb4+? does nothing for Black and in fact causes them to lose a tempo after 3.c3! A check given with the sole intention of delaying an inevitable defeat by one move is referred to as a "spite check", and may be considered somewhat unsporting (Eade 2005:65).
There are many instances, however, when checking the opponent's king may be a useful tactic or part of a tactic, either in attacking or in defense. Checking is often used in combinations with many other tactics or simply to force an opponent into a position where the opposing king can be checkmated, otherwise taken advantage of, or is otherwise worse for the opponent. Some attacks involve numerous checks to force an opponent into a losing position, especially when the king is exposed. An unexpected check in a forced combination or an overlooked cross-check in a planned series of checks may serve as sort of a zwischenzug, foiling the plan.
Some uses of checking:
In early Sanskrit chess (c. 500-700) the king could be captured and this ended the game. The Persians (c. 700-800) introduced the idea of warning that the king was under attack (announcing check in modern terminology). This was done to avoid the early and accidental end of a game. Later the Persians added the additional rule that a king could not be moved into check or left in check. As a result, the king could not be captured (Davidson 1949:22).
In this usage, the words "check" and "chess" come via Arabic from Persian shāh, meaning "king" or "monarch" (Murray 2012:159).