Doubled pawns

Pawns of the same color on the same file. Doubled pawns are often a real weakness, especially in the endgame. Still, masters will accept doubled pawns in their own camp for definite gains in return. Above all, doubling opens a file that the side with the doubled pawns can often seize and put to good use. Such structures already come up in the opening: the Nimzowitsch Defense, the Exchange Variation of the Ruy Lopez, some lines of the Caro-Kann Defense, and others. Doubled pawns are weaker when they're also isolated. But even here there are exceptions: doubled, isolated pawns in the center sometimes control important squares.