Key square

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Chess xol45.svg Chess xod45.svg Chess xol45.svg

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.


King and pawn versus king[edit]

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[edit]

Key squares with rook pawn
a b c d e f g h
8
Chessboard480.svg
f8 black cross
g8 black circle
h8 black cross
f7 black cross
g7 black circle
h7 black cross
h6 black cross
e5 black king
h5 white pawn
8
7 7
6 6
5 5
4 4
3 3
2 2
1 1
a b c d e f g h
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 "X".

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 on the right. 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 "X", 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[edit]

a b c d e f g h
8
Chessboard480.svg
f5 black circle
g5 black circle
h5 black circle
b4 black circle
c4 black circle
d4 black circle
g3 white pawn
c2 white pawn
8
7 7
6 6
5 5
4 4
3 3
2 2
1 1
a b c d e f g h
Dots indicate key squares for a pawn on the second and third ranks
a b c d e f g h
8
Chessboard480.svg
f7 black circle
g7 black circle
h7 black circle
b6 black circle
c6 black circle
d6 black circle
f6 black circle
g6 black circle
h6 black circle
g5 white pawn
c4 white pawn
8
7 7
6 6
5 5
4 4
3 3
2 2
1 1
a b c d e f g h
Key squares for a pawn on the fourth and fifth ranks
a b c d e f g h
8
Chessboard480.svg
b8 black circle
c8 black circle
d8 black circle
f8 black circle
g8 black circle
h8 black circle
b7 black circle
c7 black circle
d7 black circle
f7 black circle
g7 white pawn
h7 black circle
c6 white pawn
8
7 7
6 6
5 5
4 4
3 3
2 2
1 1
a b c d e f g h
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[edit]

a b c d e f g h
8
Chessboard480.svg
a8 black king
c8 black cross
c7 white king
b6 white pawn
8
7 7
6 6
5 5
4 4
3 3
2 2
1 1
a b c d e f g h
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 on the right, with the white king on either the square indicated or the square marked by "x", the position is stalemate if Black is to move.

Example from game[edit]

Gligorić vs. Fischer, 1959
a b c d e f g h
8
Chessboard480.svg
c8 black king
a6 black circle
b6 black circle
c6 black circle
b4 white pawn
c4 white king
8
7 7
6 6
5 5
4 4
3 3
2 2
1 1
a b c d e f g h
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[edit]

a b c d e f g h
8
Chessboard480.svg
a8 black king
a6 white circle
b6 black pawn
c6 white circle
d6 white circle
e6 white circle
a5 black circle
b5 white pawn
c5 black circle
d5 black circle
e5 black circle
h2 white king
8
7 7
6 6
5 5
4 4
3 3
2 2
1 1
a b c d e f g h
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).
a b c d e f g h
8
Chessboard480.svg
c7 white king
d7 black cross
b6 white circle
c6 white circle
d6 white circle
e6 black pawn
f6 white circle
g6 white circle
h6 white circle
b5 black circle
c5 black circle
d5 black circle
e5 white pawn
f5 black circle
g5 black circle
h5 black circle
f4 black cross
g4 black king
8
7 7
6 6
5 5
4 4
3 3
2 2
1 1
a b c d e f g h
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 "x"). 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 in zugzwang)
2... Kg6
3. Kxe6 and White wins (de la Villa 2008:173).

Example with a protected passed pawn[edit]

Walker, 1892
a b c d e f g h
8
Chessboard480.svg
c6 black king
b4 white pawn
c4 black pawn
d4 white circle
e4 white circle
f4 white circle
c3 white pawn
c2 white king
8
7 7
6 6
5 5
4 4
3 3
2 2
1 1
a b c d e f g h
Black prevents the white king from reaching a key square

In this example, White would win if his king can get to any of the key squares (marked by the white dots). 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[edit]

a b c d e f g h
8
Chessboard480.svg
f8 white circle
f7 white circle
g7 black king
b6 white king
f6 black pawn
e5 black pawn
g5 black pawn
e4 white pawn
g4 white pawn
f3 white pawn
8
7 7
6 6
5 5
4 4
3 3
2 2
1 1
a b c d e f g h
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[edit]

Jan Drtina, 1908
a b c d e f g h
8
Chessboard480.svg
f8 black king
b5 black circle
c5 black circle
d5 black circle
c3 white pawn
d1 white king
8
7 7
6 6
5 5
4 4
3 3
2 2
1 1
a b c d e f g h
White gets to a winning position by getting to the key square at 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 on the right, the key squares for the white king are b5, c5, and d5. Black can prevent the white king from reaching a key square directly, e.g.:

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) 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[edit]

Notes[edit]

References[edit]