Lesson 12

X-Wing

The rule

When a number is locked to the same two columns in two rows (or to the same two rows in two columns), that candidate is removed everywhere else in those two crossing lines.

Step by step

1. In each base row, 4 appears as a candidate in only two cells. These cells line up in the same two column positions, forming an X-Wing!
2. The X-Wing means 4 must occupy one of the two diagonal pairs of these four corners. So in the two columns that hold them, we can remove the candidate 4 from every cell outside the two base rows.