Doubled pawns
Two pawns of the same colour on the same file — created by a capture and often considered a structural weakness.
Doubled pawns are two pawns of the same colour sitting on the same file. They arise from a capture: a pawn captures a piece sideways and ends up alongside its own pawn, or — more commonly — a pawn is taken and the recapturing pawn moves to the same file as another friendly pawn.
The classical evaluation treats doubled pawns as a weakness. They cannot defend each other; only one can advance; the file in front of them is often fixed; and they limit the pieces that should be supported by pawn breaks. The doubled c-pawns that arise after Bxc3 bxc3 in the Nimzo-Indian are the most famous example, and they have shaped opening theory for a century.
But doubled pawns also bring resources. The half-open file beside them is available to the rook, which often produces real pressure. The doubled c-pawn after bxc3 in the Nimzo-Indian gives White a strong centre and an open b-file for the rook — compensation that has been debated for decades.
The most damaging case is isolated doubled pawns: two pawns on the same file with no pawns on either adjacent file. These are nearly always a liability. The standard examples are after exchanges in the Caro-Kann Exchange Variation or after certain Ruy Lopez sequences where Black ends up with doubled c-pawns and an isolated structure.