Hidden singles in Sudoku: find the only place for a digit
Look across one complete row, column, or 3×3 box. Choose a missing digit and mark every cell that can still contain it. If the digit appears in only one candidate list, that cell is forced.
Definition
A hidden single is a digit that has only one possible cell in a row, column, or 3×3 box, even when that cell still shows other candidates.
When to use it and what to verify
When to look for one
After checking cells for naked singles, choose one missing digit and follow it across an entire row, column, or 3×3 box.
Required condition
The digit must be missing from the unit and legal in exactly one of its empty cells. That cell may still show other candidates.
Common mistake
Do not judge only the target cell. A hidden single is proved by the absence of that digit from every other cell in the unit.
One digit has one possible cell in the unit.
- 1. Follow candidate 7Scan every candidate list in row 4.
- 2. Find its only locationOnly R4C6 contains candidate 7. The cell also contains 6, so it is not a naked single.
- 3. Place the hidden singleBecause row 4 needs a 7 and no other cell can take it, R4C6 must be 7.
Candidate 7 appears only at R4C6 in row 4, so R4C6 = 7.
A reference verified by Doku.su
Doku.su checks every placement or elimination in this example with its deterministic Sudoku engine. Illustrative candidates are labelled when they do not represent a complete puzzle state.
Published · Updated
Report a correction
Liked this? Try the next step on a real grid.
doku.su has more free lessons like this one.
Each lesson teaches one idea at a time.