X-ray defences are where the piece is defended indirectly by X-ray through an attacking piece. You can use X-ray defences against Rooks, Bishops, Queens, but not against Knights. These tactics don’t occur very often, so are less important for beginners to learn, but are important in advanced chess.
Diagram: Bishop X-ray Defends Rook
There is a common X-ray defence in back rank mating combinations that involve two rooks (or a Rook and Queen). In the diagram, taking Rooks will be checkmate, but returning the Re4-e8, will be protected via X-ray by the other Rook, and there’s no checkmate.
Diagram: X-ray Defence with Two Rooks
In another example, White thinks there’s a win by an amazing Queen sacrifice combination. But Black can defend by X-ray. Can you see Black’s saving move? Hint: don’t take the Queen!
Diagram: X-ray Defence with Queen
Note that there isn’t really anything in chess called an “X-ray attack”. That would probably be called a “skewer”.
Related Chess Tactics
Read more about these related chess tactics and strategies: