Peak Sudoku

X-Wing

A digit restricted to exactly two cells in two different rows, forming a rectangle.
All techniques

Overview

Tier 4
Fish, wings, chains & uniqueness
Typical difficulty
Peak Score 55–70
Also known as
2-Fish, Fish Pattern, Conjugate Pair Rectangle

An X-Wing occurs when a candidate digit appears exactly twice in two different rows, and those candidate cells align in the same two columns, allowing eliminations across both columns.

The X-Wing is the most famous advanced Sudoku technique and the foundation of all 'fish' patterns (2-fish). It bridges the gap between single-house logic and grid-wide geometric deductions. When a digit is forced into one of two positions in two parallel lines (e.g. rows), and those positions line up across the same perpendicular lines (columns), they form a rectangle with diagonal options. Whichever diagonal contains the true digit, each column receives exactly one occurrence, meaning that digit can be completely eliminated from all other cells in those columns.

Worked examples

1 / 5
Reading the exampleCells forming the patternCells losing candidatesEliminated candidates
In columns 2 and 4, the digit 2 can only go in rows 4 and 8. The two columns take one 2 in each of those rows, so 2 can be removed from the rest of rows 4 and 8.
Removes: 2 from R4C6, R4C5, R8C1, R8C5

How to apply it

Spotting the pattern and making the deduction, in order.

  1. Pick a candidate digit and find two different rows where that digit appears in exactly two cells.
  2. Check that the candidate cells in both rows line up in the exact same two columns, forming a rectangle.
  3. In the first row, the digit must go in either the left corner or the right corner.
  4. If it lands in the left corner of the first row, it is forced into the right corner of the second row (and vice versa).
  5. Either diagonal pair forces exactly one placement in each of the two columns.
  6. Safely erase that candidate from all other cells along both columns.

Common mistakes

  • Allowing a third candidate in one of the base rows. The base rows MUST have exactly two candidate cells.
  • Eliminating from the base rows instead of the crossing columns.
  • Assuming the four cells must be in the same box. In fact, an X-Wing never sits in a single box (that would just be locked candidates).

Frequently asked questions

Can an X-Wing be column-based?
Yes! If two columns have the target candidate in exactly two rows, that candidate can be eliminated from all other cells in those two rows.
Does an X-Wing immediately place a number?
Usually not directly. It eliminates candidates from the intersecting columns, which frequently uncovers naked singles or pointing pairs.
What is the difference between an X-Wing and a Swordfish?
An X-Wing is a 2-line fish (2 rows, 2 columns). A Swordfish is a 3-line fish (3 rows, 3 columns). The underlying logic is identical.
Peak Score 60–75SwordfishThe 3-row / 3-column generalisation of an X-Wing.Peak Score 65–80Finned X-WingAn X-Wing plus one or two extra candidates (the fin) inside one box.Peak Score 60–75SkyscraperTwo rows (or columns) where a digit has exactly two positions, sharing one column (or row).

Put X-Wing into practice

Try a puzzle from the Hard tier (Peak Score 51–60).

Play a Hard puzzleAll 25 techniques