Hidden Single
Within one house, a digit has only a single cell it can legally occupy — even though that cell may still show other candidates.
Two rows already have a 7, blocking four cells in this box.
How to spot it
- 1Pick a digit and a house (row, column, or box).
- 2Mark every cell in that house where the digit is still allowed.
- 3If exactly one cell qualifies, the digit belongs there.
When to use
Your first scan of any puzzle. Hidden singles place more digits than any other technique.
Common mistake
Only checking cells with one candidate. Hidden singles hide inside cells that still look busy.
Worked Example
Look at the top-right box. A 7 already sits in one row crossing it, ruling out two cells; a 7 in another row rules out more; a 7 in one column and another in a second column narrow it further. Add them up and every cell in the box is blocked from holding a 7 except one.
That surviving cell might still show several other candidates pencilled in — it doesn't look empty or obvious at a glance. But since no other cell in the box can take a 7, that cell must be the one. That's what makes it "hidden": the single is disguised by candidates for other digits, and only shows up once you check, digit by digit, where each one can actually go.
From the Blog
- Sudoku for Beginners: How to Solve Your First Puzzle, Step by StepNew to Sudoku? Learn the three rules, your first moves, and solve an easy puzzle step by step — with worked grid diagrams and no experience needed.
- The 5 Sudoku Techniques Every Beginner Should Learn FirstSkip the overwhelm. Master these five techniques — scanning, naked singles, hidden singles, cross-hatching, and pointing pairs — with worked grid diagrams for each.