The X-Wing Technique in Sudoku
Stuck on a hard grid where scanning and pairs have run dry? The X-Wing is the first “advanced” Sudoku technique most solvers learn — a rectangle of four candidates that lets you eliminate a digit elsewhere. It looks intimidating but rests on one simple idea, and once you can see the rectangle you will spot it everywhere. This guide shows you exactly what an X-Wing is, how to find one, and why the elimination is always safe.
How to use the X-Wing, step by step
- Pencil in candidates first — the X-Wing works on candidate marks, so fill every empty cell with its possible digits before you hunt for it.
- Pick one digit to chase — choose a number such as 5 and look at it across the whole grid, one digit at a time.
- Find two rows where the digit has only two spots — scan for a row where that digit can go in exactly two cells, no more.
- Check the columns line up — if a second row has the same digit limited to the same two columns, the four cells form a rectangle: that is an X-Wing.
- Eliminate the digit from those two columns — remove it as a candidate from every other cell in those two columns, outside the rectangle.
- Look for new singles — the eliminations usually expose a naked or hidden single, restarting your solve.
What is an X-Wing?
An X-Wing is a pattern where a single digit is restricted to the same two columns in two different rows, forming the four corners of a rectangle. Because the digit must end up on one diagonal of that rectangle or the other, it can be removed as a candidate from the rest of those two columns.
In the excerpt below, the digit 5 can only go in columns B or F in both Row 2 and Row 5 — four cells making a rectangle:
| Col B | Col F | |
| Row 2 | 5 | 5 |
| Row 5 | 5 | 5 |
Whichever diagonal the 5s take, columns B and F each receive exactly one 5 from these two rows — so a 5 anywhere else in column B or F can be crossed out.
How to spot an X-Wing
Hunt one digit at a time and look for two rows where that digit has only two candidate cells. If the two cells in both rows share the same pair of columns, you have an X-Wing. The same works the other way round: two columns where the digit has only two spots that line up on the same rows.
- Always work from full pencil marks — an X-Wing is invisible without candidates.
- Scan the rows for the digit first, then the columns; it is faster than checking both at once.
- The two rows need not be adjacent — Row 2 and Row 8 count just as much as Row 2 and Row 3.
New to candidates and singles? Start with Sudoku tips & strategy, then come back — the X-Wing only pays off once the basics are second nature.
Why the elimination is always safe
Inside the rectangle the digit can sit on only one of the two diagonals, and either way each of the two columns receives the digit exactly once. That guarantees the digit is used up in those columns by the rectangle, so it cannot also appear elsewhere in them. You are not guessing — you are proving where the digit cannot be.
X-Wing vs Swordfish
The Swordfish is the X-Wing’s bigger sibling: the same idea stretched from two rows and two columns to three. If you can read an X-Wing, the Swordfish technique is a small step up — a digit confined to three columns across three rows, eliminating it from the rest of those columns. Learn the X-Wing solidly first; the pattern recognition transfers directly.
Common mistakes
- Forgetting it must be exactly two cells. If a row has the digit in three candidate cells, it is not an X-Wing for that row.
- Mismatched columns. The two rows must share the same two columns — close is not enough.
- Eliminating in the wrong direction. A row-based X-Wing eliminates from the two columns; a column-based one eliminates from the two rows.
- Reaching for it too early. Exhaust singles, pairs and pointing pairs first — the X-Wing is rarely needed on Easy or Medium boards.
X-Wing in Sudoku: FAQ
What is an X-Wing in Sudoku?
An X-Wing is a solving pattern where one digit is limited to the same two columns in two different rows, forming a rectangle. Because the digit must take one diagonal of that rectangle, it can be eliminated from the rest of those two columns. It works identically with rows and columns swapped.
How do you spot an X-Wing?
Work from full pencil marks and chase one digit at a time. Find two rows where that digit has exactly two candidate cells, and check whether those cells fall in the same two columns. If they do, the four cells are an X-Wing and you can remove the digit from those columns elsewhere.
When should you use the X-Wing?
Reach for the X-Wing only after singles, naked and hidden pairs, and pointing pairs have stopped making progress — typically on Hard, Expert and Evil puzzles. On easier grids the simpler techniques solve everything, so the X-Wing is rarely needed.
Is the X-Wing an advanced technique?
It is usually the first advanced technique solvers learn, sitting just above pairs and pointing pairs. The idea is simple once you see the rectangle, but it does require pencil marks and a little practice to spot quickly.
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. Both confine a digit to a grid of intersections and eliminate it from the rest of those lines. If you understand the X-Wing, the Swordfish is a natural next step.