Weak Squares

A weak square is a square that no pawn can defend any more. Weak squares are good landing squares for enemy pieces.

A pawn move weakens squares

Black plays g6. The pawn on g6 no longer defends f6 and h6, so those squares are weak.

FEN: rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

a b c d e f g h
8 r n b q k b n r 8
7 p p p p p p p p 7
6 . . . . . . . . 6
5 . . . . . . . . 5
4 . . . . . . . . 4
3 . . . . . . . . 3
2 P P P P P P P P 2
1 R N B Q K B N R 1
  a b c d e f g h

A knight uses a weak square

White knight is on e5. Black pawns have moved past the squares that could have attacked it, so e5 is strong for White.

FEN: 4k3/8/8/4N3/3P4/8/8/4K3 w - - 0 1

a b c d e f g h
8 . . . . k . . . 8
7 . . . . . . . . 7
6 . . . . . . . . 6
5 . . . . N . . . 5
4 . . . P . . . . 4
3 . . . . . . . . 3
2 . . . . . . . . 2
1 . . . . K . . . 1
  a b c d e f g h

Weak squares near the king

White king has castled and Black has played g6. The squares f6 and h6 near the king are weak and need piece defence.

FEN: rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

a b c d e f g h
8 r n b q k b n r 8
7 p p p p p p p p 7
6 . . . . . . . . 6
5 . . . . . . . . 5
4 . . . . . . . . 4
3 . . . . . . . . 3
2 P P P P P P P P 2
1 R N B Q K B N R 1
  a b c d e f g h

Key points

A pawn move can weaken squares. A pawn that moves forward no longer defends the squares behind it.
A square is weak when no friendly pawn defends it and no friendly pawn can move to defend it.
Enemy pieces love weak squares. A knight on a weak square is hard to drive away.
Avoid making pawn moves that weaken squares near your king. Those squares become targets.
Attack a weak square with pieces, not pawns. Pieces can land there and stay there.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Rust Playground

Exercise

Black plays g6. Which square becomes weak because the pawn no longer defends it?

f6 f5 g5 h5