X-Wing

Find a digit that appears as a candidate in exactly two cells of one row, and exactly two cells of another row, with all four cells sharing the same pair of columns. They form the corners of a rectangle.

Whichever way it resolves, the digit takes one corner in each row — and therefore one cell in each of the two columns. It follows that the digit cannot appear anywhere else in those two columns, and can be rubbed out of them.

This is the first technique that reasons across the whole grid rather than inside a single unit, and it is the usual dividing line between a puzzle a beginner can finish and one they cannot. It works identically with rows and columns swapped.