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 defenses. X-ray captures are covered in a separate level.

Examples

White's queen on b7 attacks Black's knight on a6.
Running away with the knight to b4 or b8 would leave the bishop on c7 unprotected and allow Qxc7.
Protecting the knight with the rook on a8, by moving Nc8, would leave the rook unprotected and allow Qxa8.
1...Qc8 is the only good move. It x-ray-defends the knight on a6, because Qxa6 can now be answered with Qxa6.

An x-ray defense is sometimes used to defend against a fork.
Here black’s queen on d4 is attacking both of White's rooks (a fork), and 1.Qe5 is the only move to defend both.

How to spot an X-ray defense

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.