When the power of a piece, either to attack or to defend, seems to pass through an intervening enemy piece. The X-ray attack, also known as a skewer, involves two pieces of the same color both caught in the same line of attack along a diagonal, rank, or file. The attacking piece forces the first and more valuable piece to move out of the way, which allows the second piece to be captured. As a defensive tactic the X-ray occurs when one piece is defended by another piece, through an attacking enemy piece standing between the two.