X-Wing

An X-Wing is a pattern of one digit and four cells at the corners of a rectangle. If the digit has exactly two candidate cells in each of two rows, and those cells lie in the same two columns, then the digit must occupy two opposite corners of the rectangle. It can therefore be removed from every other cell of those two columns. The same works with rows and columns swapped. It is the first Expert technique in Bare Sudoku.

When to use it

When singles, locked candidates and subsets are exhausted. The X-Wing is the first Expert pattern to look for and often appears when a digit has few candidates left, typically with four to six copies already placed. It needs complete notes for the digit you examine.

How to spot it

Choose a digit and look for rows in which it has exactly two candidate cells. If two such rows share the same two columns, check those columns for other candidates of the digit: those are the eliminations. Then repeat with columns as the base. In the game, tap the digit key: every candidate for it is highlighted, and a rectangle of four highlighted notes is easy to see.

Example

X-Wing
c1c2c3c4c5c6c7c8c9
r13868368912475
r2149758263
r325734634646189
r447826781681234634693751478
r5928512347146134148
r647316846592147
r76783814914751342
r8571456238967
r935792145681467371341467

Bold digits are given, blue digits were placed while solving, small digits are candidates. Highlighted cells form the pattern, crossed-out candidates are removed, and a circled candidate is the digit to place.

  1. Digit 7 has exactly two candidate cells in row 6 and exactly two in row 8, and they line up in the same two columns, 1 and 9: r6c1, r6c9, r8c1, r8c9.
  2. Row 6 puts its 7 in column 1 or 9, and row 8 puts its 7 in the other one. Either way, columns 1 and 9 each receive a 7 from these two rows.
  3. So 7 cannot appear anywhere else in columns 1 and 9. Remove 7 from r4c1, r4c9, r9c1, r9c9.

Common mistakes

In Bare Sudoku

Needed from level: Expert
In the game, the hint says: X-Wing

Play this puzzle · Play Sudoku · Sudoku solver

Practice puzzles

In these puzzles, X-Wing is the hardest technique you need. Each link opens the puzzle in the game; the solver link shows every step.

  1. Expert 3.2 · Sudoku solver
  2. Expert 3.2 · Sudoku solver
  3. Expert 3.2 · Sudoku solver
  4. Expert 3.2 · Sudoku solver
  5. Expert 3.2 · Sudoku solver
  6. Expert 3.2 · Sudoku solver
  7. Expert 3.2 · Sudoku solver
  8. Expert 3.2 · Sudoku solver
  9. Expert 3.2 · Sudoku solver
  10. Expert 3.2 · Sudoku solver

Frequently asked questions

What is an X-Wing in Sudoku?

A digit that has exactly two candidate cells in each of two rows, in the same two columns. The digit occupies two opposite corners of that rectangle, so it is removed from the rest of the two columns. Rows and columns can be swapped.

Why does an X-Wing work?

Each of the two rows must place the digit in one of the two columns, and the two rows cannot use the same column. So the two columns each receive their copy of the digit from these rows, and every other candidate in those columns is impossible.

Is an X-Wing a fish?

Yes. The X-Wing is the fish of size two; Swordfish is the fish of size three. Bare Sudoku finds both.

Which level has X-Wings?

Expert or Master. Bare Sudoku rates a puzzle as Expert when it needs an X-Wing or another advanced pattern, such as the Y-Wing or the Skyscraper, and as Master when it also needs chains or other techniques beyond this guide.

All techniques

· Updated