One Choice

An empty cell has exactly one legal candidate after checking its row, column and box. Place that letter.

Use it on your board

1. Choose a row, column or box to inspect. A “unit” means any one of these.
2. Check legal candidates against every placed letter. Pencil notes are reminders, not proof: missing or stale notes can hide a possibility.
3. Verify every condition in the rule above before making an elimination. A pattern that almost matches is not enough.
4. 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.