The X-Wing
The X-Wing is the first of the advanced techniques. It looks at one digit across two rows (or two columns) at once, and when that digit is boxed into a rectangle, the rest of the rectangle's lines lose it.
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In short
- In two rows, a digit fits in only two cells each, and in the same two columns.
- Those four cells are the corners of a rectangle. The digit takes two opposite corners.
- Either way, each of the two columns gets its copy from those corners, so strike the digit from the rest of both columns.
- The same works with columns and rows swapped.
An example
Here the X-Wing runs down columns, so it clears rows. Either the 9s sit at row 3, column 3 and row 8, column 7, or at row 8, column 3 and row 3, column 7. Both ways, row 3 and row 8 have their 9 inside the rectangle, and three candidates go at once.
How to find one
- Pick one digit and look at it alone across the board. Writing every cell's notes helps.
- Find rows where the digit has exactly two places. Then look for a second such row with the same two columns.
- Do the same with columns.
- Check that something is left to strike. An X-Wing with nothing outside it changes nothing.
Why it works
Each of the two base lines needs the digit once, and it can only take it in the two shared lines. Two copies, two cross lines, one per cross line: so each cross line already has its copy among the corners, and nowhere else on it can hold one.
Bigger fish
The same idea across three lines is the Swordfish, and across four the Jellyfish. They are rarer and harder to see, but the reasoning doesn't change.
Common mistakes
- Three places in a row. Each base row must have the digit in exactly the two shared columns, no third cell.
- Striking from the base lines. The candidates go from the other two lines, never the ones you started from.
- Missing notes. An unwritten candidate can make a false X-Wing look real.
Questions
What is an X-Wing in Sudoku?
A pattern where one digit, in two rows, fits only in the same two columns (or in two columns, only in the same two rows). The digit can then be removed from the rest of those columns (or rows).
Why is it called an X-Wing?
The two possible placements cross the rectangle diagonally, like an X.
When do I need an X-Wing?
On hard puzzles, after singles, pairs and locked candidates have run out.
Next: Swordfish · Y-Wing · all techniques
Drill the X-Wing with the Smith
Ninesmith Sudoku is a calm Sudoku app for iPhone. Its Pattern Trainer drills the X-Wing in sessions of 30 to 90 seconds, and in your own puzzles the Smith names the technique and shows the pattern before anything goes in.
Get the appThe full X-Wing drills are part of Premium; the first scenario is free. No ads in any tier.