Key squares
In chess, particularly in endgames, a key square is a square such that if a player's king can occupy it, he te can force some gain such as the promotion of a pawn or the capture of an opponent's pawn.
King and pawn versus king
The key squares are relative to the position of the pawn. Whether or not the white king can reach a key square depends on the position of the pieces. Of course, even if the white king occupies a key square, accurate play is still required in order to promote the pawn (Müller & Lamprecht 2007:20–22).
Rook pawn
| a | b | c | d | e | f | g | h | ||
| 8 |      | 8 | |||||||
| 7 | 7 | ||||||||
| 6 | 6 | ||||||||
| 5 | 5 | ||||||||
| 4 | 4 | ||||||||
| 3 | 3 | ||||||||
| 2 | 2 | ||||||||
| 1 | 1 | ||||||||
| a | b | c | d | e | f | g | h | ||
A rook pawn (on the a-file or h-file) has much less chance of promoting than other pawns. The reason is that if the opposing king can get to any square in front of the pawn, it can not be driven away from the file, and the pawn can not queen. Black can always draw if he can reach the c8 square for an a-pawn (pawn on the a file) or the equivalent f8 for an h-pawn, except for the position in the next diagram, with White to move. Therefore, an advanced rook pawn generally has two key squares: b7 and b8 for an a-pawn, and g7 and g8 for an h-pawn. The key squares are indicated by the black dots in the position in the diagram on the right:
| a | b | c | d | e | f | g | h | ||
| 8 |  | 8 | |||||||
| 7 | 7 | ||||||||
| 6 | 6 | ||||||||
| 5 | 5 | ||||||||
| 4 | 4 | ||||||||
| 3 | 3 | ||||||||
| 2 | 2 | ||||||||
| 1 | 1 | ||||||||
| a | b | c | d | e | f | g | h | ||
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 (Silman 2007:105–6).
The pawn can also promote in the position on the right (if White is to move), after
- 1. h7
However, in practice most of the time the black king can stop a rook pawn because it is usually close enough that the white king can not prevent it from getting in front of the pawn (or capturing it).
Other pawns
| a | b | c | d | e | f | g | h | ||
| 8 | | 8 | |||||||
| 7 | 7 | ||||||||
| 6 | 6 | ||||||||
| 5 | 5 | ||||||||
| 4 | 4 | ||||||||
| 3 | 3 | ||||||||
| 2 | 2 | ||||||||
| 1 | 1 | ||||||||
| a | b | c | d | e | f | g | h | ||
Pawns other than rook pawns have a much better chance of promoting. 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, for example see the diagram on the right:
| a | b | c | d | e | f | g | h | ||
| 8 | | 8 | |||||||
| 7 | 7 | ||||||||
| 6 | 6 | ||||||||
| 5 | 5 | ||||||||
| 4 | 4 | ||||||||
| 3 | 3 | ||||||||
| 2 | 2 | ||||||||
| 1 | 1 | ||||||||
| a | b | c | d | e | f | g | h | ||
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 diagram on the right, for example:
| a | b | c | d | e | f | g | h | ||
| 8 | | 8 | |||||||
| 7 | 7 | ||||||||
| 6 | 6 | ||||||||
| 5 | 5 | ||||||||
| 4 | 4 | ||||||||
| 3 | 3 | ||||||||
| 2 | 2 | ||||||||
| 1 | 1 | ||||||||
| a | b | c | d | e | f | g | h | ||
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.
Once White's king occupies a key square he can keep the opposing king from blocking the advance of the pawn, as will be shown below (Müller & Lamprecht 2007:16–18).
| a | b | c | d | e | f | g | h | ||
| 8 | | 8 | |||||||
| 7 | 7 | ||||||||
| 6 | 6 | ||||||||
| 5 | 5 | ||||||||
| 4 | 4 | ||||||||
| 3 | 3 | ||||||||
| 2 | 2 | ||||||||
| 1 | 1 | ||||||||
| a | b | c | d | e | f | g | h | ||
There is an exception to the key squares rule with a knight pawn, the black king in the corner, and Black 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.
| a | b | c | d | e | f | g | h | ||
| 8 | | 8 | |||||||
| 7 | 7 | ||||||||
| 6 | 6 | ||||||||
| 5 | 5 | ||||||||
| 4 | 4 | ||||||||
| 3 | 3 | ||||||||
| 2 | 2 | ||||||||
| 1 | 1 | ||||||||
| a | b | c | d | e | f | g | h | ||
It is important to emphasize that White wins if he gets his 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 White are b5, c5, and d5. However, 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, see below).
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).
Blocked pawns
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. With White to move:
- 1. Kg3! Kb7
- 2. Kf4 Kc7
- 3. Ke5 Kd7
- 4. Kd5 Kc7
- 5. Ke6 The white king reaches a key square.
- 5.... Kc8
- 6. Kd8 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).
See also
References
- de la Villa, Jesús (2008), 100 Endgames You Must Know, New in Chess, ISBN 978-90-5691-244-4
- Müller, Karsten; Lamprecht, Frank (2007), Secrets of Pawn Endings, Gambit Publications, ISBN 978-1-904600-88-6
- Template:Harvard reference