What is an X-Wing in sudoku?

An X-Wing is a pattern where a candidate appears in exactly two cells in each of two different rows, and those cells line up in the same two columns. This forms a rectangle whose corners hold the candidate. Because the candidate must occupy opposite corners of that rectangle, you can eliminate it from the rest of those two columns.

How do I spot an X-Wing?

Pick a candidate and find two rows where it appears in only two cells each, then check if those cells share the same two columns. If they do, you have a row-based X-Wing and can eliminate the candidate from those columns outside the rectangle. The same works swapping rows and columns for a column-based X-Wing.

Why does an X-Wing let me eliminate candidates?

In the rectangle, the candidate must go in one diagonal pair of corners or the other, since each row allows only those two positions. Either way, both affected columns end up with the candidate placed at a corner. That guarantees the candidate cannot live anywhere else in those two columns, so those cells can be cleared.

What is the difference between an X-Wing and a Swordfish?

An X-Wing uses two rows and two columns with a candidate confined to two cells per line, while a Swordfish extends the same idea to three rows and three columns. Swordfish cells do not all need to be filled; each defining line just needs the candidate limited to the relevant columns or rows. Swordfish is essentially a larger, harder-to-spot generalization of the X-Wing.

X-Wing: Sudoku Technique with Examples

X-wing is the first technique that spans the whole grid for a single digit. It is also the simplest of the fish family. Get comfortable with it and swordfish and jellyfish follow the same shape.

How it works

Pick a digit X. Find two rows where X has exactly two candidate cells. If the four candidate cells across those two rows line up across the same two columns, you have an X-wing.

Each of the two rows must place X in one of its two candidate cells. Whichever way the puzzle resolves, X will occupy a diagonal pair of the four rectangle corners: top-left and bottom-right, or top-right and bottom-left.

Either way, each of the two columns gets exactly one X at a rectangle corner. So X cannot live anywhere else in those two columns. Erase it from every other cell in those two columns.

Digit 5 is locked to 2 rows → erase 5 from the crossing columns.

When to look for it

After pointing pairs and box-line reduction stop producing eliminations. The candidate sets need to be tight, so always make sure pencil marks are clean before you scan.

Track digits that have only two or three candidate cells per row. Those are the digits worth checking. Anything with four or more candidate cells per row is not X-wing fuel.

Step-by-step example

Pick a digit, say 6. List the rows where 6 has exactly two candidate cells remaining.
For each pair of such rows, check whether the two candidate columns are the same in both rows.
When the columns match, you have an X-wing.
Erase the digit from every other cell in those two columns.
Re-scan for singles. X-wings usually unlock one immediate placement that cascades.

Tips for spotting it

Pick digits that already appear in seven or eight of the nine rows. Their remaining homes are tightly constrained.
Use a finger or cursor to trace columns across the two candidate rows. Misaligned columns kill the pattern.
X-wings produce eliminations in the columns (or rows, if pivoted), never in the four corner cells.
Re-mark the affected columns after the eliminations. Singles often appear immediately.

Common mistakes

Counting cells with more than two candidates for the digit. X-wing requires exactly two per row.
Eliminating in the rows. The eliminations sweep the two columns.
Confusing X-wing with a naked pair. Naked pair sits inside one unit; X-wing spans two rows and two columns.
Skipping the pivot. Half of all X-wings live in columns.

Practise it

Hard sudoku contains an X-wing more often than you would think. Mark candidates fully, walk every digit through the rows, and look for two rows with matching two-cell candidate columns. Once X-wing is reflex, move on to swordfish.

X-Wing