Naked Singles and Hidden Singles
Two ways a cell can only hold one number — and how to spot them quickly.
A single is a cell whose value can be determined with certainty. There are two flavours, and knowing both doubles your progress.
Naked single
A naked single is a cell where eight of the nine digits are already present in its row, column or box. Only one candidate remains, so it must be the answer.
Hidden single
A hidden single is a digit that can only go in one cell within a row, column or box, even though that cell may have other candidates. Scanning usually reveals these.
Tip
Turn on notes mode in Quietgrid (press N) and pencil in candidates for a busy box. Singles tend to jump out once notes are visible.