The X-Wing is one of the four Expert-level techniques on Sudoku-Land, alongside the Skyscraper, the Swordfish and the XY-Wing. Unlike the Hard-level techniques, which stay inside a single unit (one row, one column or one box), the X-Wing relies on a pattern spread across the whole grid — which makes it look intimidating at first. In practice, once the principle clicks, it becomes very quick to spot.
If one candidate digit appears in exactly two positions in each of two rows, and those positions sit in the same two columns, then that digit can be eliminated from every other cell in those two columns. (The same reasoning works identically with rows and columns swapped.)
Those four cells form a rectangle — hence the name X-Wing, after the cross shape drawn by the rectangle's diagonals.
An X-Wing only becomes visible once candidate notes have been pencilled in across the whole grid (see the full guide if you need it). To hunt for one, work through three conditions, in order:
If all three conditions hold, candidate 5 can be removed from every other cell in those two columns — including cells in boxes far away from the original rectangle.
Here is a schematic example (digits already placed are not shown — only candidate 5 matters here). In this grid fragment, candidate 5 appears only in columns 3 and 7, on rows 2 and 6 — the four cells in yellow:
Rows 2 and 6, columns 3 and 7: candidate 5 is confined to the four yellow corners. It can therefore be removed from the other cells in those two columns (in red) — even outside rows 2 and 6.
In practice, once those four eliminations are made, it often becomes possible to solve one or more cells outright elsewhere in the grid — that domino effect is what makes the technique so useful.
The reasoning is a proof by contradiction. Take row 2: candidate 5 is only possible in column 3 or column 7. So there are two scenarios, and exactly one of them is true:
Either way, each column (3 and 7) gets its own 5 — one on row 2, the other on row 6. It doesn't matter which scenario is the right one: in both, those two columns already have "their" 5 reserved on those two rows. No other cell in those columns can hold a 5.
An X-Wing is easier to spot on a digit that already looks "rare" in the grid — a candidate that only appears in a small number of cells overall. Going through the candidates one at a time (1, then 2, then 3…) and looking only at their positions row by row, rather than scanning the whole grid at once, makes the pattern show up much faster.
Once the X-Wing is under your belt, two techniques at the same level are direct variations on it: the Skyscraper (an "asymmetric" X-Wing, with a single shared column instead of two) and the Swordfish (the same principle extended to 3 rows × 3 columns). All twelve techniques used on the site, with an example for each, are covered on the How to play page, and the summary table by difficulty level lives on Techniques by level.