A pawn chain is a diagonal line of pawns where each one defends the next; the front pawn is the head and the undefended rear pawn is the base.
Pawns protect each other along a diagonal: every pawn in the chain is guarded by the one behind it — except the pawn at the very back, the base, which no pawn defends. The pawn at the front, the head, points the way the chain wants to expand.
The classic guidance (from Aron Nimzowitsch) is to attack the chain at its base — the one link no pawn can defend. Knock out the base and the pawns ahead of it are left hanging. You usually do this by pushing a pawn break against it.
Interlocking chains set the whole strategy of openings like the French and King’s Indian: each side knows which end of the board to play on by looking at which way their chain points.
At its base — the rear pawn that no other pawn defends. Break it with a pawn push and the pawns in front of it lose their support and become weak.
The head is the most advanced pawn (it points the direction of your play); the base is the rearmost pawn, the only link not defended by another pawn — and therefore the target.
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