X-Wing

The pattern

Choose a digit. Find a row where it has exactly two possible cells. Find a second row where it also has exactly two, and where those two cells sit in the same columns as the first pair. The four cells form the corners of a rectangle.

Whatever happens, one of the two rows takes the digit in the left column and the other takes it in the right column. So both columns already have their copy of the digit, and it can be erased from every other cell in those two columns.

Rows or columns, both directions work

The same pattern read the other way round is just as valid: two columns that each have exactly two places for the digit, aligned on the same two rows, eliminate that digit from the rest of those rows. When you hunt, check both orientations — players who only ever scan rows miss half the X-Wings on the board.

How to hunt for one

Work one digit at a time across the whole grid. For that digit, note every row where it has exactly two candidate cells. If two of those rows share their column pair, you have found it. Digits that are already placed five or six times are the best candidates, because their remaining rows are tightly constrained.

This is a scan you cannot do cell by cell. That is the real reason X-Wings feel invisible: the search pattern is different from everything used at lower levels.

Why it is valid

Suppose an outside cell in one of the two columns took the digit. That column is then finished with it, which forces both rows to place the digit in the other column — two copies in one column, which is illegal. The rectangle is a self-contained contradiction, which is why no guessing is involved.

Where it shows up here

Master puzzles on this site require exactly one advanced step, and the X-Wing is the most common one to appear. Extreme puzzles require at least two, usually mixing an X-Wing with a different advanced pattern.