Doubled pawns
Doubled pawns are two of one side's pawns stacked on the same file, usually after a capture, and they tend to be a structural weakness because neither can guard the other.
The usual cause is a recapture after a trade of pieces, for instance a pawn taking a knight and landing on a file where one of its own pawns already stands. The stacked pair has real drawbacks. The two pawns cannot guard each other, the front one can block the rear one, and together they cover fewer squares than two pawns on separate files would. If the capture also leaves a neighbor without support, the weakness spreads.
Yet the structure often comes with offsetting gains. The capture can open a file for a rook, add control of a central square, or arrive together with the bishop pair. Judge it by the full exchange, not by the pawn count alone.
Against doubled pawns, fix them in place, attack the front pawn, and use the squares they no longer watch. With them, keep pieces on the board, use the opened file, and look for a chance to trade the weaker of the two. In an endgame the flaw is hardest to hide, so be wary of simplifying.
Avoid refusing a favorable capture only because it doubles your pawns; a bishop pair or an open file can be worth far more than tidy pawns.
A neighboring idea is Pawn island. A pawn island is a cluster of one side's pawns on neighboring files, separated from that side's other pawns by a file without any of its pawns, and fewer islands usually signal a healthier structure.
Strategic ideas such as "Doubled pawns" show up in your games as small decisions, move after move. A first lesson with George Li reviews those decisions and turns them into a study plan.
Related terms
An isolated pawn is a pawn with no friendly pawn on either neighboring file, so no other pawn can ever defend it and the square in front of it is a lasting weakness.
A weak pawn is a pawn that is hard to defend with other pawns and so must be protected by pieces, making it a long-term target for the opponent.
A pawn island is a cluster of one side's pawns on neighboring files, separated from that side's other pawns by a file without any of its pawns, and fewer islands usually signal a healthier structure.
Connected pawns are two or more friendly pawns standing on adjacent files, side by side or on neighboring ranks, so that they can support each other as they advance.
A pawn majority is a larger number of pawns on one wing or in one area of the board than the opponent has there, giving the chance to create a passed pawn.
A backward pawn is a pawn that has fallen behind the pawns on neighboring files and cannot safely advance, because the square in front of it is controlled by an enemy pawn.
Guides that go further
What to decide first, the questions to ask, what a first lesson should show and the red flags to avoid.
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A game review, a read on your level and a written study plan from George Li.
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A first lesson starts with your games: George reviews them and builds a study plan around what he finds.