Isolani - Weakness

Weakness

In the endgame, isolated pawns are a weakness in pawn structure because they cannot be defended by other pawns as with connected pawns. In this diagram, the white pawn on the e4 square and the black pawn on a7 are isolated.

Isolated pawns are weak for two reasons. First, the pieces attacking them usually have more flexibility than those defending them. In other words, the attacking pieces enjoy greater freedom to make other threats (win pieces, checkmate, etc.), while the defending pieces are restricted to the defense of the pawn. This is because a piece that is attacking a pawn can give up the attack to do something else, whereas the defending piece must stay rooted to the spot until the attacking piece has moved. The defending piece is thus said to be "tied down" to the pawn.

The second reason is that the square immediately in front of the isolated pawn is weak, since it is immune to attack by a pawn (often providing an excellent outpost for a knight). Thus an isolated pawn provides a typical example of what Wilhelm Steinitz called weak squares.