What is an X-wing in sudoku? The trick that breaks expert puzzles

Most easy and medium sudokus fall to two techniques: cross-hatching (placing a digit in a box where it has only one legal cell) and naked singles (a cell with only one legal digit). On harder puzzles, you eventually hit a wall: maybe 25-30 cells left, every empty cell has two or three candidates, no single placement is forced.

The X-wing breaks that wall. It doesn’t place a digit directly. Instead, it eliminates candidates in cells far away from the pattern, which then unlocks the placements you need to make progress.

Pick a digit — say, 4. Look at the candidate-marks for 4 across the whole grid. You’re hunting for two rows where 4 can only go in exactly two cells, and those two cells share the same pair of columns.

That’s the X-wing: four cells forming a rectangle. In each of the two rows, 4 has to be in one of the two corners. We don’t know which corner of which row, but we know one thing for certain: 4 will appear in both of those columns, once each, in those two rows.

Here’s the trick. In row 3, the 4 has to be in column 2 or column 7 — those are the only options. In row 7, same thing. But we don’t need to know which is which to make a useful deduction. Just consider both possibilities:

• Case A: row 3’s 4 is in column 2. Then row 7’s 4 can’t also be in column 2 (column rule). So row 7’s 4 is in column 7.
• Case B: row 3’s 4 is in column 7. Then row 7’s 4 can’t be in column 7. So row 7’s 4 is in column 2.

Either way, columns 2 and 7 each get exactly one 4 from these two rows. That means every other cell in columns 2 and 7 can’t be a 4 — because both column-4s are already accounted for. Go ahead and rub out 4 as a candidate from every other cell in those two columns.

Often, those eliminations leave a cell with only one remaining candidate (a naked single), which gets you moving again.

The brute approach is to scan one digit at a time:

1. Pick a digit, 1 through 9.
2. For each row, count how many cells could still be that digit. If any row has exactly two, jot down which columns.
3. Look for two rows whose “exactly two” columns are identical. If you find them, you’ve found an X-wing on that digit.
4. Eliminate the digit from every other cell in those two columns.

The same pattern works flipped 90°: two columns where the digit can only go in two cells each, sharing the same pair of rows. Eliminate from the rest of those two rows. Most expert-puzzle X-wings show up in just one of these orientations per puzzle, but check both.

You will not spot an X-wing without pencil marks. The pattern is invisible in your head — even strong solvers don’t hold “which two cells in row 3 can be a 4” in working memory across all nine digits. So: fill in candidates aggressively on harder puzzles, scan by digit, and trust the pattern when you see it.

On Melio, the in-game pencil-mark toggle lets you flip into note-mode with a single keystroke (or button tap on mobile). Use it freely on Expert and Extreme; you’re not cheating, you’re using the right tool.

The X-wing is the smallest fish: 2 rows × 2 columns. The family extends:

• Swordfish: 3 rows × 3 columns. A digit appears as a candidate in 2 or 3 cells across three rows, with those candidates confined to the same three columns. Same elimination logic, applied to all three columns.
• Jellyfish: 4 rows × 4 columns. Rare in practice; if you spot one, you’re probably solving a custom Extreme.

The progression is clean. Once you internalize the X-wing, the others fall out naturally. We have a separate guide for those →

A Y-wing (sometimes called XY-wing) is unrelated despite the name. The Y-wing is a three-cell pattern based on bivalue cells — cells with exactly two candidates each — where the values are arranged so that whichever one of the three is right, a fourth cell can’t hold a specific digit.

X-wing = a digit, four cells, a rectangle. Y-wing = a chain of three cells with overlapping candidate pairs. Different patterns, different reasoning. Y-wing has its own guide.

X-wings show up reliably on Expert and Extreme puzzles. If you want to drill the technique, queue up a few Expert solos, fill candidates aggressively, and hunt for two-cell rows on each digit before placing anything. Once you find your first X-wing in the wild, you’ll start seeing them everywhere.