Hard

X-Wing

What it is

A digit that fits only in the same two columns in two rows (or the reverse) can be removed from those columns everywhere else.

How to spot it

Pick a digit. Find two rows where it has exactly two possible cells, in the same two columns. The four cells form a rectangle.

Example

Basis
Removed / placed

In rows 2 and 6, 3 can only go in columns 2 and 7. Either r2c2 and r6c7 or r2c7 and r6c2 are 3, so 3 is removed from the rest of columns 2 and 7 – here r4c2 and r8c7.

Used from level: Hard