Key square

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In chess, particularly in endgames, a key square (also known as a critical square) is a square such that if a player's king can occupy it, he can force some gain such as the promotion of a pawn or the capture of an opponent's pawn. Key squares are useful mostly in endgames involving only kings and pawns. In the king and pawn versus king endgame, the key squares depend on the position of the pawn and are easy to determine. Some more complex positions have easily determined key squares while other positions have harder-to-determine key squares. Some positions have key squares for both White and Black.

Chess Strategy board game

Chess is a two-player strategy board game played on a chessboard, a checkered gameboard with 64 squares arranged in an 8×8 grid. The game is played by millions of people worldwide. Chess is believed to be derived from the Indian game chaturanga some time before the 7th century. Chaturanga is also the likely ancestor of the Eastern strategy games xiangqi, janggi, and shogi. Chess reached Europe by the 9th century, due to the Umayyad conquest of Hispania. The pieces assumed their current powers in Spain in the late 15th century with the introduction of "Mad Queen Chess"; the modern rules were standardized in the 19th century.

In chess and chess-like games, the endgame is the stage of the game when few pieces are left on the board.

King (chess) piece from the board game chess

In chess, the king (♔,♚) is the most important piece. The object of the game is to threaten the opponent's king in such a way that escape is not possible (checkmate). If a player's king is threatened with capture, it is said to be in check, and the player must remove the threat of capture on the next move. If this cannot be done, the king is said to be in checkmate, resulting in a loss for that player. Although the king is the most important piece, it is usually the weakest piece in the game until a later phase, the endgame. Players cannot make any move that places their own king in check.

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King and pawn versus king

In an endgame with a king and pawn versus a king, the key squares are relative to the position of the pawn. Assume that White has the pawn. If the white king can occupy a key square, he can force the promotion of the pawn but accurate play is required. Whether or not the white king can reach a key square depends on the position of the pieces and which player is to move ( Müller & Lamprecht 2007 :20–22).

Rook pawn

Key squares with rook pawn
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Dots are key squares for a rook pawn. In addition, Black stops the pawn if the black king gets to any of the squares marked with "×".

An advanced rook pawn generally has two key squares: the two squares on the adjacent file that touch the promotion square, i.e. b7 and b8 for a white a-pawn, and g7 and g8 for a white h-pawn. The key squares are indicated by the black dots in the position in the diagram. If White's king can reach either of the two key squares, he can keep Black's king away and the pawn will promote. If the Black king can reach any of the squares marked with a dot or an "×", it stops the pawn – either by blocking the pawn or preventing the white king from reaching a key square ( Silman 2007 :105–6).

Other pawns

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Dots indicate key squares for a pawn on the second and third ranks
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Key squares for a pawn on the fourth and fifth ranks
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Key squares for a pawn on the sixth and seventh ranks

Pawns other than rook pawns have more key squares. If the pawn is on the second, third, or fourth rank , there are three key squares – the square two squares in front of the pawn and the squares to the left and right of that square. The key squares are indicated by the black dots in the diagrams above. If the pawn is on the fifth or sixth rank, there are six key squares: the square in front of the pawn and the squares to the left and right, as well as the square two squares in front of the pawn, and the squares to the left and right of it, see the middle diagram. When the pawn is on the seventh rank, the key squares are the squares on the seventh and eighth rank that touch the pawn's square (see the diagram on the right).

An easy way to remember the key squares is to note that if the pawn is not beyond the midpoint of the board, there are three key squares that are two ranks ahead. If the pawn is on the fifth or sixth rank there are six key squares on the two ranks in front of the pawn. If the pawn is on the seventh rank, the adjoining squares on the seventh and eighth ranks are key squares ( Müller & Lamprecht 2007 :16–18).

An exception

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Exception to key squares - stalemate with Black to move if the white king is on c7 or c8

There is an exception to the key squares rule with a knight pawn on its sixth rank, the defending king in the corner, and the defender to move. In the diagram, with the white king on either the square indicated or the square marked by "×", the position is stalemate if Black is to move.

Stalemate

Stalemate is a situation in the game of chess where the player whose turn it is to move is not in check but has no legal move. The rules of chess provide that when stalemate occurs, the game ends as a draw. During the endgame, stalemate is a resource that can enable the player with the inferior position to draw the game rather than lose. In more complex positions, stalemate is much rarer, usually taking the form of a swindle that succeeds only if the superior side is inattentive. Stalemate is also a common theme in endgame studies and other chess problems.

Example from game

Gligorić vs. Fischer, 1959
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Position after 57.Kc4. Marked squares are key squares; Black draws.

This position from a game [1] between Svetozar Gligorić and Bobby Fischer illustrates key squares. Black to move can keep the white king from reaching a key square by 57...Kb8, so the game is drawn ( Müller & Lamprecht 2007 :20). If the white king moves to the fifth rank , Black takes the opposition. (See Opposition (chess)#Example for more details of this game.)

Blocked pawns

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Key squares for blocked pawns (white dots for the white king; black dots for the black king)

In a position with a blocked pair of pawns (opposing pawns on the same file , the key squares for a player's king extend for three files on either side of the opponent's pawn. In this position, the first king to reach one of his key squares will win the opponent's pawn and protect his own. Even though the white king is farther away from the pawns, White wins if he moves first:

1. Kg3! Kb7
2. Kf4 Kc7
3. Ke5 Kd7
4. Kd5 Kc7
5. Ke6

The white king reaches a key square.

5... Kc8
6. Kd6 Kb7
7. Kd7 Kb8
8. Kc6 Ka7
9. Kc7 Ka8
10. Kxb6 and White wins (see king and pawn versus king endgame) ( de la Villa 2008 :172–73).
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White to move, does not go directly to a key square

When both kings can reach a key square, a position of mutual zugzwang may occur. The first king to attack the opposing pawn must save a square for attack and defense (the squares marked "×"). With White to move:

1. Kd7!

The only winning move; all other moves lose. For instance, if 1.Kd6?? then 1...Kf5 puts White in zugzwang and Black wins.

1... Kf5
2. Kd6!

Now Black is in zugzwang.

2... Kg6
3. Kxe6 and White wins ( de la Villa 2008 :173).

Example with a protected passed pawn

Walker, 1892
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Black prevents the white king from reaching a key square

In this example, White would win if his king could get to any of the key squares (marked by the white dots). But Black is able to prevent this and draw the game – with or without the move. For example:

1. Kd2 Kd5
2. Ke3 Ke5

The only move to draw.

3. Kf3 Kf5

The only move to draw.

4. Kg3 Ke5
5. Kg4 Ke4

The only move to draw ( Müller & Lamprecht 2007 :52).

Example with more pawns

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The squares with white dots and the f6-square are key squares for White. White to move wins; Black to move draws.

In this example, f6 is also a key square for the white king. White to move wins; Black to move draws. (All of Black's moves are the only move to draw.)

1... Kh6!!
2. Kc7 Kg7
3. Kb7 Kh7
4. Kb8 Kh8
5. Kc8 Kg8
6. Kd7 Kh7
7. Ke6 Kg6!( Müller & Lamprecht 2007 :95–96).

Any key square by any route

Jan Drtina, 1908
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White gets to a winning position by getting to the key square b5.

With a king and pawn versus a lone king, it is important to get the attacking king to any key square and the path to a key square is not always direct. For instance, in the diagram, the key squares for the white king are b5, c5, and d5. Black can prevent the white king from reaching a key square directly, for example:

1. Kd2 Ke7
2. Kd3 Kd7
3. Kc4 Kc6

Taking the opposition; however, the white king can reach a key square (b5) by going on the other side of the pawn:

1. Kc2! Ke7
2. Kb3 Kd6
3. Kb4 Kc6
4. Kc4

Opposition, and Black is in zugzwang.

4... Kd6
5. Kb5

or

4... Kb6
5. Kd5

and the white king has occupied a key square and has a winning position ( Müller & Lamprecht 2007 :20).

See also

Related Research Articles

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Tarrasch rule in chess, general principle abouot chess middlegames and endgames that rooks should be placed behind passed pawns;  stated by Siegbert Tarrasch (1862–1934)

The Tarrasch rule is a general principle that applies in the majority of chess middlegames and endgames. Siegbert Tarrasch (1862–1934) stated the "rule" that rooks should be placed behind passed pawns – either yours or your opponent's. The idea behind the guideline is that (1) if a player's rook is behind his passed pawn, the rook protects it as it advances, and (2) if it is behind an opponent's passed pawn, the pawn cannot advance unless it is protected along its way.

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Bishop and knight checkmate chess endgame with a king, bishop, and knight against a lone king, which can almost always be checkmated with perfect play in at most 33 moves

The bishop and knight checkmate in chess is the checkmate of a lone king which can be forced by a bishop, knight, and king. With the stronger side to move and with perfect play, checkmate can be forced in at most thirty-three moves from any starting position where the defender cannot quickly win one of the pieces. The exceptions occur when (1) the defending king may be forking the bishop and knight so that one of them is lost on the next move, or (2) the knight may be trapped in a corner by the defending king and the knight is lost in one or two moves, and the position is not in the "stalemate trap". These exceptions constitute about 0.5% of the positions. Checkmates are possible with the defending king on any square at the edge of the board, but can be forced only from positions with different material or if the defending king is in a corner controlled by the bishop or on a square on the edge next to a corner, but mate adjacent to the corners not controlled by the bishop is only two moves deep, so is not generally encountered unless the defending side plays inaccurately. Although this is classified as one of the four basic or elementary checkmates, it occurs in practice approximately only once in every 6,000 games.

Rook and pawn versus rook endgame

The rook and pawn versus rook endgame is of fundamental importance to chess endgames, ,, , and has been widely studied, . Precise play is usually required in these positions. With optimal play, some complicated wins require sixty moves to either checkmate, win the defending rook, or successfully promote the pawn. In some cases, thirty-five moves are required to advance the pawn once.

In chess, opposition is the situation occurring when two kings face each other on a rank or file, with only one square in-between them. In such a situation, the player not having to move is said to "have the opposition". It is a special type of zugzwang and most often occurs in endgames with only kings and pawns. The side with the move may have to move the king away, potentially allowing the opposing king access to important squares. Taking the opposition is a means to an end and is not always the best thing to do.

The chess endgame of a queen versus pawn is usually an easy win for the side with the queen. However, if the pawn has advanced to its seventh rank it has possibilities of reaching a draw, and there are some drawn positions with the pawn on the sixth rank. This endgame arises most often from a race of pawns to promote.

Corresponding squares in chess occur in some chess endgames, usually ones that are mostly blocked. If squares x and y are corresponding squares, it means that if one player moves to x then the other player must move to y in order to hold his position. Usually there are several pairs of these squares, and the members of each pair are labeled with the same number, e.g. 1, 2, etc. In some cases they indicate which square the defending king must move to in order to keep the opposing king away. In other cases, a maneuver by one king puts the other player in a situation where he cannot move to the corresponding square, thus the first king is able to penetrate the position. The theory of corresponding squares is more general than opposition, and is more useful in cluttered positions.

The opposite-colored bishops endgame is a chess endgame in which each side has a single bishop, but the bishops reside on opposite-colored squares on the chessboard, thus cannot attack or block each other. Without other pieces these endings are notorious for their tendency to result in a draw. These are the most difficult endings in which to convert a small material advantage to a win. With additional pieces, the stronger side has more chances to win, but not as many as if the bishops were on the same color.

A pawnless chess endgame is a chess endgame in which only a few pieces remain and none of them is a pawn. The basic checkmates are types of pawnless endgames. Endgames without pawns do not occur very often in practice except for the basic checkmates of king and queen versus king, king and rook versus king, and queen versus rook. Other cases that occur occasionally are (1) a rook and minor piece versus a rook and (2) a rook versus a minor piece, especially if the minor piece is a bishop.

The rook and bishop versus rook endgame is a chess endgame where one player has just a rook, bishop and king, and the other player has only a rook and king. It has been studied many times through the years. This combination of material is one of the most common pawnless chess endgames. It is generally a theoretical draw, but the rook and bishop have good winning chances in practice because the defense is difficult. Ulf Andersson won the position twice within a year, once against a grandmaster and once against a candidate master; and grandmaster Keith Arkell has won it 18 times out of 18. In positions that have a forced win, up to 59 moves are required. Tony Kosten has seen the endgame many times in master games, with the stronger side almost always winning. Pal Benko called this the "headache ending".

In chess endgames with a bishop, a pawn that is a rook pawn may be the wrong rook pawn. With a single bishop, the result of a position may depend on whether or not the bishop controls the square on the chessboard on which the pawn would promote. Since a side's rook pawns promote on opposite-colored squares, one of them may be the "wrong rook pawn". This situation is also known as having the wrong-colored bishop or wrong bishop, i.e. the bishop is on the wrong colored squares in relation to the rook pawn. In many cases, the wrong rook pawn will only draw, when any other pawn would win. A fairly common defensive tactic is to get into one of these drawn endgames, often through a sacrifice.

Réti endgame study chess endgame study about king and pawn against king and pawn

The Réti endgame study is a chess endgame study by Richard Réti. It was published in 1921 in Kagans Neueste Schachnachrichten. It demonstrates how a king can make multiple threats and how it can take more than one path to a given location, using the same number of moves. It is covered in many books on the endgame. The procedure is known as the "Réti Maneuver" or "Réti's Idea", ,. Endgame composer Abram Gurvich called the theme "The Hunt of Two Hares" and it appears in many other studies and games. It is also called "chasing two birds at once".

Queen and pawn versus queen endgame

The queen and pawn versus queen endgame is a chess endgame in which both sides have a queen and one side has a pawn, which they are trying to promote. It is very complicated and difficult to play. Cross-checks are often used as a device to win the game by forcing the exchange of queens. It is almost always a draw if the defending king is in front of the pawn.

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