An advanced technique where a digit's candidates in two rows are confined to the same two columns, allowing elimination from those columns' other cells.

X-Wing applies when a specific digit's candidate positions in two rows (or columns) are limited to the same two columns (or rows). The digit can then be eliminated from all other cells in those two columns. Required for Master-level puzzles.

Recognizing the Pattern

Four candidate cells form a rectangle, with diagonals creating an X shape - hence the name X-Wing. The digit will occupy two of these four cells along one diagonal (which diagonal is undetermined). In either case, the corresponding columns will contain the digit, so it can be eliminated from other cells.

Extension to Swordfish

Expanding X-Wing (2x2) to 3x3 gives Swordfish, and to 4x4 gives Jellyfish. The logical basis is the same, but the increased pattern complexity makes them significantly harder to spot.