Sudoku technique
The X-Wing
The first technique that reasons across the whole grid rather than inside one row, column or box. Four cells at the corners of a rectangle, and an elimination that holds without ever solving any of them.
What an X-Wing is
An X-Wing is four cells at the corners of a rectangle. You need two rows in which some digit has exactly two possible homes, and those homes have to sit in the same two columns. When that happens, the digit can be removed from those two columns everywhere else on the grid.
It works in either orientation — two columns whose candidates line up on the same two rows eliminates along those rows instead.
Why the elimination holds
Take the two rows. Each of them has to contain the digit somewhere, and each has only two places available. Whichever way it resolves, the two rows use one of the two columns each — they cannot both take the same column, because then one row would have no home left.
So between them, those two rows consume the digit in both columns. Any other cell in those columns is finished, without you ever finding out which corner is which.
It is the same reasoning as a naked pair, stretched across the grid instead of confined to one unit. A pair says “these two cells own these two digits”. An X-Wing says “these two rows own this digit in these two columns”.
Step 1 of 5
A real position
Part way through a hard grid. Every simpler move has been played out, and the 4s are what is left to reason about.
How to spot one
X-Wings do not appear until candidates are marked and the simple moves are gone, so this is never your first look at a board. When you get there:
Pick a digit. Go down the rows and note only the ones where that digit has exactly two candidate cells — usually there are two or three such rows out of nine. Then check whether any two of them share both columns. Repeat per digit, then repeat with columns and rows swapped.
Digits already placed five or six times are the best candidates, because they have few homes left and are far more likely to be pinned to two per line.
When it is worth the effort
Honestly: on hard and expert grids, and rarely anywhere else. Easy and medium puzzles are built to fall to hidden singles and pairs, and hunting rectangles on them wastes time you could spend solving.
In a timed match the calculation is harsher still. If a board has got hard enough to need an X-Wing, the cells that decided the match were usually taken several minutes earlier.
The next rung up is the Swordfish, which is the same idea across three lines instead of two.
Other techniques
Try it on a real board
The free board on the home page has a notes mode, which is what every technique past hidden singles needs. Then take the same eye into a live match.