Sudoku

Sudoku X-Wing — How to Find and Use It

5 min read  ·  Burmly

The X-Wing is one of those patterns that looks complicated on paper but becomes instantly obvious once you see it in a real puzzle. It removes candidates, not places numbers directly — which is why it matters in harder puzzles where basic scanning stops working.

What the X-Wing actually is

Find a digit that appears as a candidate in exactly two cells in each of two different rows. If those two rows share the same two columns — you have an X-Wing.

In that situation, the digit can be eliminated from all other cells in those two columns.

Example — looking for digit 5: Row 2: · · [5] · · · [5] · · (columns 3 and 7) Row 6: · · [5] · · · [5] · · (columns 3 and 7) → Remove 5 from all other cells in column 3 and column 7

Why this elimination is valid

There are only two possible placements for 5 in row 2: column 3 or column 7. The same is true for row 6. Because the rows share the same columns, one of these must happen:

Either way, columns 3 and 7 each contain exactly one 5 from these two rows. Any other 5 candidate in column 3 or 7 is impossible — eliminate it.

The shape: If you draw lines connecting the four candidate cells, you get a rectangle. That's why it's called X-Wing — the diagonals form an X.

How to spot it while solving

X-Wings don't jump out at you. You need to be actively looking for them. The practical approach:

  1. Pick a digit you haven't placed much of — say, 7
  2. Scan each row and note which rows have exactly 2 candidate cells for 7
  3. Check if any two of those rows share the same pair of columns
  4. If yes — eliminate 7 from all other cells in those columns

Then do the same scan by column instead of row. An X-Wing can run in either direction.

Column-based X-Wing

The logic is identical, just rotated. Find a digit that appears in exactly two cells in each of two columns, and those two columns share the same two rows. Eliminate that digit from all other cells in those rows.

Example — digit 3: Col 4: rows 1 and 8 each have exactly one [3] candidate Col 9: rows 1 and 8 each have exactly one [3] candidate → Remove 3 from all other cells in row 1 and row 8

When X-Wing actually appears

Mostly in hard and expert-rated puzzles. Easy and medium puzzles rarely need it — hidden singles and naked pairs get you through. If you're stuck on a hard puzzle and no simpler technique is working, scanning for X-Wings on the digits you've placed least is a reliable next step.

X-Wing vs. Swordfish

Swordfish is the three-row version of the same idea. Instead of two rows and two columns, it involves three rows and three columns. The elimination logic is the same — just more cells. If X-Wing isn't breaking through, Swordfish is the next thing to look for on the same digit.

Practice X-Wing live

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