Explanation

X-rays are similar to batteries in that your pieces can attack and defend “through” other pieces. With a battery, your long-range pieces (queens, rooks, and bishops) can attack and defend “through” your own pieces. With an x-ray, your long-range pieces can attack and defend “through” your opponent’s pieces. An x-ray defense is a move that protects your piece “through” the attacking piece. This is much rarer than a normal protection, but sometimes it is the only possible defense. An x-ray capture is a move that captures an opponent’s piece. After the capture, the capturing piece is x-ray defended.

In the first diagram, the white rooks aren’t protecting each other directly but are protecting each other “through” the black rook. Black could not win material by capturing either white rook, because the other white rook could then recapture.

In the first diagram, the white rooks aren’t protecting each other directly but are protecting each other “through” the black rook. Black could not win material by capturing either white rook, because the other white rook could then recapture.

In the third diagram, White can win a rook with the x-ray capture indicated by the arrow. The move then results in the same position as shown in the first diagram, where the two white rooks protect each other with an x-ray.

In this level we are focusing on x-ray captures. X-ray defenses are covered in a separate level.

Examples

An x-ray capture is a winning trade that uses an x-ray.
This is easiest to explain with some examples:

It looks like Black's queen on f5 is only attacked once by White's queen.
However, there is an x-ray of White's rook on f1.
Indeed, after White captures on f5 with 1.Qxf5, the queen of f5 is x-ray protected by the rook on f1.
After Black recaptures with 1...Rxf5, there is no longer a rook on f4, and White can win the rook and the game with 2.Rxf5.

X-ray captures can also happen "through" pawns:

Black just played Ra1, threatening to capture the rook with Rxd1 as well as promoting the pawn with b1=Q.
The rook on a1 is only attacked by the White's rook on d1. However, there is an x-ray by White's bishop on g7. Therefore, White can win with the x-ray capture 1.Rxa1. The rook on a1 is now x-ray defended by the Bishop.
After Black recaptures with 1...bxa1=Q, there is no longer a blocking pawn on b2, and White can capture the new queen with 2.Bxa1.
White is two pawns up in the resulting endgame and should win.

How to spot an X-ray capture

An x-ray doesn’t happen so often, and it can be easy to overlook. Practicing identifying x-rays will help you be aware of this option in your own games.