The X-Wing Technique identifies a pattern where a specific candidate appears in exactly two columns of the same two rows, or in exactly two rows of the same two columns. When this pattern is identified, the candidate can be eliminated from all other cells in those columns or rows.
- Row-based X-Wing: If a candidate appears in exactly two matching columns across two different rows, this candidate can be eliminated from all other cells in those columns.
- Column-based X-Wing: If a candidate appears in exactly two matching rows across two different columns, this candidate can be eliminated from all other cells in those rows.
In this puzzle, a Row-based X-Wing occurs where the number 7 appears in exactly two columns (columns 2 and 9) in rows 6 and 7. As a result, the number 7 can be eliminated from all other cells in columns 2 and 9.
This puzzle combines Hidden and Naked Singles along with the X-Wing technique for an engaging challenge.
How to spot it
Find a digit that, in two different rows, is a candidate in only the same two columns. The four cells form a rectangle. The same works with rows and columns swapped.
When to use it
It is the gateway advanced technique; reach for it when singles, subsets, and Omission stall and you have good pencil marks.
Common mistakes
- Eliminating in the wrong direction. A row-based X-Wing removes the digit from the two columns, not the two rows.
- Accepting a near-miss where one of the rows has the digit in a third column; the candidate must be confined to exactly two columns in both rows.
Frequently asked questions
- What is an X-Wing in Sudoku?
- An X-Wing is a pattern where a candidate digit appears in only two cells of two different rows, and those cells share the same two columns, forming a rectangle. The digit can then be eliminated from the rest of those two columns.
- How do you find an X-Wing?
- Choose a digit and look for two rows in which it is a candidate in exactly the same two columns. Those four cells form the X-Wing, and you eliminate the digit elsewhere in those columns. The same applies with rows and columns reversed.
- What is the difference between an X-Wing and a Swordfish?
- An X-Wing uses two rows and two columns. A Swordfish extends the same logic to three rows and three columns, where the digit is confined to the same three columns across three rows.