Doubled pawns are two pawns of the same colour stacked on one file, the result of a capture that pulled a pawn sideways onto a file it already occupied.
Doubled pawns are usually a small weakness: they can’t defend each other, they move slowly, and the rear pawn can get stuck behind the front one, making the whole group easy to blockade.
They’re not always bad, though. Capturing toward the centre often opens a file for your rook and gives you extra control of the squares the pawns now guard — a fair trade in many openings.
Doubled and isolated pawns at the same time are the genuinely weak version. As a guide: accept doubled pawns when you get an open file or the bishop pair for them, and avoid them when you get nothing in return.
Not always. They can be, but if you got an open file, extra central control, or the bishop pair in return, they’re often a price worth paying.
They arise when you recapture with a pawn along a file you already have a pawn on — pulling a second pawn onto the same column.
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