The X-Wing is the first technique that reads two houses against two others. Take one digit: if two columns each have exactly two cells left that can hold it, and those cells sit in the same two rows, the four cells form a rectangle. The digit must occupy one diagonal of that rectangle — either both “\” corners or both “/” corners — which is where the X gets its name.
Either diagonal covers both rows. So each of those two rows gets its copy of the digit at one of the rectangle’s corners — and the digit can be struck from every other cell of both rows. (The same logic runs transposed: two rows pinning two columns.)
To spot one, track a single digit across the grid and look for columns reduced to two candidate cells. When two such columns line up row-for-row, the rectangle is set and the eliminations follow by pure logic — no guessing at which diagonal wins.
On the board
Follow the digit 1 on this board. Column 2 has exactly two cells that can still take it — rows 2 and 6 — and column 5 has exactly the same two. The four cells R2C2, R2C5, R6C2, and R6C5 form the rectangle, and the 1s must land on one of its diagonals. Either way, row 2 and row 6 each get their 1 inside columns 2 or 5 — so the 1 is struck everywhere else along both rows: at R2C3, R2C6, and R2C9 across the top, and at R6C6 below.
This is the exact board the app's interactive X-Wing demo uses — step 23 of its 60-step walkthrough, drawn here with its live pencil marks.
