In games such as chess, shogi, and xiangqi, a check is a threat to capture the king (or general in xiangqi) on the next move turn. A king so threatened is said to be in check. On the very next move, the player whose king is in check must remove their king from check ("get out of check"), if possible. Either the threat must be stopped (by interposing a piece between the threatening piece and the king, or capturing the threatening piece) or the king must be moved to a square where it is no longer in check. If the player has no move out of check, the game ends in checkmate and the player loses.
|This article uses algebraic notation to describe chess moves.|
A check is the result of a move that places the opposing king under an immediate threat of capture by one (or sometimes two) of the player's pieces. (In some chess variants, check by more than two pieces is possible.) 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 game is over: The player whose king is checkmated loses and his opponent wins the game.
It is against the rules to make a move that puts or leaves a player's own king in check. Such a move is illegal and must be retracted (except under some rules variations of fast chess). A king cannot itself directly check the opposing king, since this would place the first king in check. (All other types of pieces can check.) However a move of the king can expose the opposing king to a discovered check.
Announcing "check" to the opponent after playing a move that gives check, is optional.
Getting out of check 
There may be up to three possible ways to get a king out of a single check on the following move:
- Capturing the checking piece, with either the king or another piece. If the checking piece is on a square next to the king, the king can capture the piece if the king does not move into a new check, i.e. if the piece is not protected by another enemy piece.
- Moving the king to an adjacent square where it will not be in check. The king is not allowed to castle when it is in check. The king may capture an enemy piece in a move to get out of check, as long as the piece is not protected.
- Blocking the check. This will only work if the checking piece is a queen, rook or bishop and there is at least one empty square in the line between this checking piece and the checked king. Blocking a check is done by moving a piece (from the checked king's army) to a square in line in between the checking piece and the checked king. (The blocking piece is then absolutely pinned to the king by the attacking piece, until it is unpinned.) (Polgar & Truong 2005:32,103).
In the position in the first diagram, White can get out of check by three methods:
- The move Nxa2, capturing the attacking piece
- Moving the king to any unattacked square (marked with "x"), i.e. Kd6, Ke5, or Ke7.
- Blocking the check by the move Rc4 or Nd5.
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. The king may, however, capture one of the checking pieces in the process, if that piece is adjacent to the king and not protected by another piece.
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.
Types of checks 
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:
- Discovered check – A discovered check is similar to any other type of discovered attack except that it is a discovered attack on the opposing king. In a discovered check, a piece moves out of the line of attack by another piece so that this other piece (which can be a queen, rook, or bishop) is then checking the opponent's king. The piece that actually moved in the discovered check move could possibly be any type of piece belonging to the same player as the checking piece except queen or the same type of piece administering the check. A discovered check could be a tactic in itself because the piece that moved could attack or otherwise create a threat to another piece on the checked king's side. The opponent has to get out of the discovered check on the following move and may not get a chance to thwart the attack by the other piece that moved.
- Double check – A double check is a check from two pieces to the opponent's king in a single move. This happens when a moved piece attacks the king, resulting in a second piece giving check by discovered check. It can also happen, though very rarely, when an en passant capture opens two lines of attack simultaneously. In algebraic chess notation, a double check move is sometimes noted with a "++" after the written move in place of the usual "+", although "++" has been used to indicate checkmate (along with "#"). A double check cannot be blocked, nor can it be met by capturing one of the checking pieces (unless the king itself makes the capture), because there is check from two directions.
- Cross-check – When a check is answered by a check, particularly when this second check is delivered by a piece blocking the first, it is termed a cross-check. In fact, a "cross-checkmate" is also possible in that way (that is, to answer a check with a checkmate) but since no such term exists, it would fall under the definition of cross-check as well. Cross-checks are rather rare but are a popular theme in chess problems.
Announcing check and notation 
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).
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).
In algebraic chess notation, a checking move is recorded like any other move, except that a "+" is normally written after the move.
Checking in tactics and strategy 
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 his 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 him 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).
However, there are many times 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 involves 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:
- Repetitive checking to prevent losing a game going poorly (to draw the game by perpetual check).
- Royal fork (knight fork of king and queen) or other forks involving the king.
- Checks to force an exchange.
- A double check could be especially bad for the opponent since there are likely to be fewer options to get out of check. A double check is often more likely to lead to checkmate or loss of material.
- A check might force a king to move so that it cannot castle later.
- Moving a piece to give check can sometimes open a line of attack on another piece. The opponent must escape the check, and therefore cannot (in general) prevent the other piece from being captured (discovered attack).
- Similarly, the piece moved to create a discovered check may attack another enemy piece, leading to the same scenario.
- The king may also, by being forced to move out of check, enable the checking piece to capture another piece (an absolute skewer).
- A check might force some piece to block it and therefore pose an absolute pin on that piece.
- A check might deflect the king from protecting some other piece (particularly in an endgame, to capture opponent's pawns).
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).
See also 
- FIDE Laws of chess
- "The King Is not Dead After All! The Real Meaning of Shah Mat or the Lesson of the Commode", Jan Newton, GoddessChess.com, September 2003
- Davidson, Henry (1949), A Short History of Chess, McKay, ISBN 0-679-14550-8 (1981 paperback)
- Eade, James (2005), Chess for Dummies (2nd ed.), Cardoza, ISBN 978-0-7645-8404-6
- Golombek, Harry (1977), Golombek's Encyclopedia of Chess, Crown Publishing, ISBN 0-517-53146-1
- Hooper, David; Whyld, Kenneth (1992), The Oxford Companion to Chess (2nd ed.), Oxford University Press, ISBN 0-19-866164-9
- Just, Tim; Burg, Daniel B. (2003), U.S. Chess Federation's Official Rules of Chess (5th ed.), McKay, ISBN 0-8129-3559-4
- Murray, H.J.R. (2012) , A History of Chess, Skyhorse, ISBN 978-1-62087-062-4)
- Polgar, Susan; Truong, Paul (2005), A World Champion's Guide to Chess, Random House, ISBN 978-0-8129-3653-7
- Schiller, Eric (2003), Official Rules of Chess (2nd ed.), Cardoza, ISBN 978-1-58042-092-1