How an X-ray works
Bishops, rooks and queens act along lines, and a piece standing in the way blocks them only for as long as it stays there. An X-ray means counting that blocked power as if it were already active, because exchanges or forced moves will soon clear the line.
There are two kinds. In an X-ray attack, a piece attacks through an enemy piece to a target behind it, which is how skewers work. In an X-ray defence, a piece defends a square through an enemy piece, so that if the enemy piece leaves the line, the defence switches on.
Where X-rays matter
They matter most when counting attackers and defenders of a square. If two pieces seem to attack and two to defend, check whether one side has a third piece acting through another. Doubled rooks, or a queen behind a rook, often decide such counts.
Back-rank combinations are a common stage: a capture on the eighth rank is recaptured by a piece that was itself standing on the attacker's line, and the line opens for a mate.
X-ray, battery and skewer
A battery is two friendly pieces on one line, such as a queen behind a bishop. An X-ray adds an enemy piece in between, or extends the line through it. A skewer is one form of X-ray attack, in which the front piece is forced to move.
Common mistakes
- Counting only the pieces that attack a square directly and missing a piece that acts through another.
- Recapturing with a piece that stands on the enemy line, and so opening it.
Questions about x-ray
What is an X-ray defence?
A piece defends a square through an enemy piece on the same line. If the enemy piece moves or is exchanged, the defence becomes active.
Is an X-ray the same as a skewer?
A skewer is one kind of X-ray attack. X-ray is the wider term and also covers defences through a piece.