Doubling rooks means placing both rooks on the same file or rank, one behind the other, so their power combines on a single line.
One rook on an open file is good; two are a battering ram. Doubled rooks defend each other, so an enemy rook cannot trade off your control of the file: if it captures the front rook, the back one recaptures and the file stays yours.
Doubling typically wins the fight for an open file. A single defending rook must eventually give way, and then the front rook invades, usually to the seventh rank, where rooks feast on pawns and pin the king to the back rank. Doubling on the seventh itself is often decisive.
The standard technique: seize the open file with one rook, meet the challenge without trading, then swing the second rook behind it. The same idea works on a rank, and adding the queen behind doubled rooks (Alekhine's gun) turns pressure into a crush.
Because they defend each other, the file cannot be traded away, and the front rook is free to invade. A lone defending rook is simply outnumbered on the line.
Often yes: in attacking formations the rook you can spare goes in front and the anchor stays behind. In file battles it matters less; what counts is that the rear rook backs up any exchange on the file.
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