X-ray attack
An x-ray is hidden pressure: a bishop, rook or queen bears on a piece or square beyond an enemy piece in its way, and the pressure turns real once the blocker moves or is exchanged.
The easiest way to understand an x-ray is to watch an exchange. Suppose your bishop looks along a diagonal at an enemy knight, and behind that knight stands the enemy queen. Nothing attacks the queen yet. But if the knight is captured or moves, the bishop's line to the queen opens at once. That unspoken threat is the x-ray, and it changes what is safe.
It matters in two ways. Capturing on the front square can turn into a trap: the defender who recaptures may find that the recapture steps onto a square still covered through the line, so it loses more than it wins. And the blocker is frozen: if it moves, the piece behind it falls.
The same idea applies to defense. A rook can defend a piece through an attacker, because once the attacker captures and is itself captured, the rook is still there. Counting exchanges without remembering x-rays leads to many mistakes.
To find x-rays, scan the lines of your long-range pieces and ask which enemy piece they are really aimed at beyond the first blocker. To avoid trouble, keep valuable pieces off lines where an enemy bishop, rook or queen waits behind a trade.
In the diagram White plays Nxe5. The bishop on b2 has been aiming at the queen on g7 through the black knight. If Black recaptures Qxe5, Bxe5 takes the queen, so Black has simply lost a knight.
It sits close to Interference. Interference is a tactic in which you land a piece on the line joining two enemy pieces that depend on each other, cutting their connection so that one of them can be won.
It sits close to Ladder mate. A ladder mate is a checkmate on the edge of the board in which two rooks, or a rook and a queen, take turns giving check on neighboring lines until the king runs out of room.
A neighboring idea is Vacating sacrifice. A vacating sacrifice gives up a piece so that it leaves, or is captured on, a square or line that another of your pieces needs, usually to deliver mate or win material.
To see how often "X-ray attack" or a pattern like it decides your own games, book a first lesson: George Li reviews your games and writes a study plan around what he finds.
Related terms
A pin ties a piece to its square: a bishop, rook or queen attacks it, and stepping aside would expose a more valuable piece standing behind it on the same line.
A skewer attacks a valuable piece that stands in front of a less valuable one on the same line. The front piece must run, and the piece behind it is then taken.
A battery is two of your pieces lined up on one file, rank or diagonal, with the rear piece backing up the front one so that both press on the same target.
In a discovered attack one of your pieces steps off a line, and that move unmasks a bishop, rook or queen standing behind it, so a single turn produces two threats.
Guides that go further
What to decide first, the questions to ask, what a first lesson should show and the red flags to avoid.
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A game review, a read on your level and a written study plan from George Li.
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