Lesson 08 of 08 — Advanced
X-Wing technique
An X-Wing is a digit that can go in only two cells of one row, and only two cells of another row, with both pairs sitting in the same two columns. Wherever the digit lands in the first row, it takes one of those two columns, so the second row is forced into the other. Every remaining cell in both columns can be cleared.
What it does
The four cells form a rectangle, and the digit has to occupy two opposite corners of it. Which pair of corners is not yet known, and does not need to be: either way, both columns are spent.
The same argument runs with rows and columns swapped, which is what makes the pattern worth scanning for in both directions. It is the first technique in the lessons that needs you to follow a single digit across the whole grid.
A worked example
Follow one digit
Pick a digit and look for rows where it has exactly two possible cells left. Two homes in a row is the shape to hunt for, so work digit by digit rather than cell by cell.
Find a matching pair of rows
Compare two of those rows. If their two cells sit in the same two columns, you have the rectangle. If the columns differ, keep looking.
Read the four corners
The digit must appear once in each row, and the two rows cannot share a column, so the four cells resolve into one of two diagonals. You do not need to know which.
Clear both columns
Remove the digit from every other cell in those two columns. The two cells of the rectangle keep it; everything else in the columns loses it.
How to spot it
- Scan for the digit, not for the cell. An X-Wing is invisible from any single cell, because each of the four cells looks unremarkable on its own.
- A digit with only two possible cells in several rows is the strongest signal. Nearly finished rows give you the pairs for free.
- Remember to try it the other way round, with two columns that each have two homes in the same two rows.
An X-Wing never places a digit, so the columns it clears have to be useful somewhere else: the eliminations should leave a single candidate, a pair or a locked candidate behind, or the pattern was correct but unhelpful. It also needs a complete candidate list. A third possible cell for the digit in either row means there is no X-Wing, and the eliminations would be wrong.
Practise it in Sudoku Mori
The game teaches every technique in this order, each with a worked example and three puzzles of its own. Lesson puzzles are kept apart from the daily ones, and any lesson can be revisited.
Keep reading
All eight: Single candidate · Hidden single · Locked candidates · Naked pair · Hidden pair · Naked triple · Hidden triple · X-Wing