X-Wing
Find two rows in which a particular letter has exactly two possible columns, the same two in both rows. Eliminate that letter from those columns in other rows. The rule also works with rows and columns swapped.
A letter-based example
A is possible only in columns 2 and 7 of rows 1 and 6. Remove A from columns 2 and 7 outside rows 1 and 6.
A BB CC D
Use it on your board
- Choose a row, column or box to inspect. A “unit” means any one of these.
- Check legal candidates against every placed letter. Pencil notes are reminders, not proof: missing or stale notes can hide a possibility.
- Verify every condition in the rule above before making an elimination. A pattern that almost matches is not enough.
- Update the affected notes, then scan for a newly forced letter. A technique often opens the way to a much simpler next move.
Classic and Mega
The candidate rule applies at both board sizes. Classic adds the hidden-line constraint; Mega uses only rows, columns and 5×5 boxes. A larger board has more candidates, so focus on one unit at a time.
These examples explain deductions, not a promise that every generated puzzle requires this technique. In particular, Mega’s launch tiers describe clue density.