Stacking two pieces on the same line

One rook on an open file is a nuisance. Two rooks on the same file is a different piece altogether, and so is a queen sitting quietly behind a bishop.

Kinds queen and bishop, queen and rook, doubled rooks
Three heavy pieces is Alekhine's gun
Works by changing the count on one square

A battery is two pieces of the same line type stacked behind each other: queen and bishop on a diagonal, queen and rook on a file, two rooks on a file, three heavy pieces on one file. The front piece does the moving. The back piece makes the front piece brave, because whatever captures it gets captured in turn.

That is the whole mechanism, and it is why a battery changes the count on a square rather than the geometry. A square you attack twice and your opponent defends once is a square you can take.

Queen behind bishop, aimed at h7

A Queen's Gambit Declined. The bishop on d3 has eight moves and one of them is Bxh7, with the queen on c2 waiting behind it.

Right now Bxh7 loses a bishop: the king takes it, and the knight on f6 covers the square as well. The battery is not a threat, it is a standing question, and every move Black makes has to answer it. The moment that f6 knight is deflected, or Black plays a move that leaves h7 defended only by the king, the bishop sacrifice becomes real.

The queen goes behind on this diagonal because the bishop is the cheaper piece and should be the one that lands first. Put the queen in front instead and you have a different weapon: the queen sacrifices itself on the square and the bishop recaptures, which is one of the standard mating patterns against a castled king.

Doubled rooks, and which one goes in front

The front rook has eight moves, and d7 is the one that matters. Nothing on the board defends it.

Once the rook reaches d7 it hits b7, which nothing defends, and it sits on the seventh rank where the second rook can join it. Black's rooks are on c8 and e8 and cannot contest the file, because a single rook opposing a doubled pair just loses the counting argument.

Doubling rooks with the queen behind them, on a file, has a name: Alekhine's gun. The order matters. Rook, rook, queen means the cheapest piece takes first. Queen, rook, rook means your queen is the one being traded off, which is usually the opposite of what you wanted.

Build the battery before you need it. Lining up takes two or three moves and your opponent gets to watch you do it, so aim it at something that cannot run away.
Count attackers and defenders on the target square, not pieces on the line. A battery is arithmetic.
A battery pointed through an enemy piece is an x-ray, and the piece in the middle is often more pinned than it looks.
Doubled rooks are how most back rank mates are actually delivered, because the first rook is allowed to be captured.

Questions people ask

What is Alekhine's gun in chess?

Two rooks doubled on a file with the queen behind them, so that three pieces bear down on one square. It is named after a game Alekhine won with the formation against Nimzowitsch in 1930. The point of the ordering is that the rooks capture first, so your opponent has to give up material to break the line rather than trading queens off it.

Which piece should go in front of a battery?

The cheaper one, if you intend to capture, because whatever it takes it gets recaptured on and you want that trade to favour you. The queen goes in front only when the plan is to sacrifice it on the target square and have the piece behind recapture, which is a mating idea rather than a material one.

Battery sits under Pieces and squares in the reference. Nearby: