Hidden Single
Within one unit, a letter is possible in exactly one cell. That cell must contain it, even when other letters are also pencilled there.
A letter-based example
Only one cell in row 4 allows A. That cell is A, even if its notes also show B and C.
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.