X-Wing
Rated 4.2 on the technique scale · A rectangle that eliminates candidates.
Take a digit that appears exactly twice in each of two rows, and in the same two columns. Those four cells form a rectangle; the digit must occupy two opposite corners, so it can be removed from every other cell in those two columns.
The same pattern works with columns as the base and rows as the target. It is the first "fish" pattern, and it is worth 4.2 points.
Swordfish is the three-line version and is worth 4.6 points. Both are common in hard puzzles.
X-Wing — 9 forms a rectangle across two rows and two columns, so it can be removed from the rest of those columns.
Practice it: x-wing shows up most often in hard and expert puzzles. Browse those levels →
How the rating uses it
When a puzzle cannot be finished with easier techniques, the solver applies X-Wing and adds 4.2 to the puzzle's score. The hardest technique a puzzle needs decides its level, and puzzles with 26 clues or fewer move up one level.