Hidden singles: find a digit’s only possible cell
A hidden single occurs when a digit has only one possible cell in a row, column or box, even if that cell has other candidates.
Updated 2026-11-13
A hidden single occurs when a digit has only one possible cell in a row, column or box, even if that cell has other candidates. Scan a unit for one digit, eliminate positions using crossing constraints, and place it in the sole remaining location.
Change the question you ask
Instead of asking what belongs in a cell, ask where a particular digit can go in a unit. A box must contain a seven. Existing sevens in crossing rows and columns rule out positions inside that box. If only one survives, seven belongs there.
This remains true when the surviving cell has candidates two, five and seven. The alternatives two and five might be legal for the cell considered alone, but using either would leave the box with nowhere to put seven.
A candidate-position example
Consider a row with three empty cells named A, B and C. Their candidate sets are:
| Cell | Candidates |
|---|---|
| A | two, five |
| B | two, five, seven |
| C | two, five |
Only B contains seven, so B is seven. You do not first have to decide which of A and C is two. The row requirement supplies the missing force.
Read the example as a candidate fragment. Other cells in the row are already filled, and none of them is seven. If a seven is already present in the row, you are not looking for another seven there.
Scan systematically
- Choose an unfinished row, column or box.
- Select a digit absent from it.
- Check every empty cell against the crossing constraints.
- If exactly one location remains, place the digit.
- Remove the placed digit from all peers and repeat nearby checks.
A box scan is visually convenient because a filled digit can exclude an entire internal row or column segment. Rows and columns deserve the same attention, however; a hidden single is not exclusively a box technique.
Why the hidden single is easy to miss
Players often keep returning to the most crowded cell. A hidden single can hide in a cell with many notes, so that habit filters out the very cell you need. Scanning candidate positions for one digit reverses the focus.
Another mistake is checking only the cells carrying written notes. An unmarked empty cell can still accept the digit. It must be tested before declaring another position unique. The deduction needs a complete view of the chosen unit.
Check the only remaining home
Try the opposite assumption in words: if the surviving cell were not seven, where would seven go in this row? If the answer is nowhere, you have the proof. This is a brief contradiction check, not a long branching guess.
Find a hidden single inside a box
Focus on the upper-left box and look only for six. In this local sketch, a dash means the cell has been ruled out for six; letters mark open positions. The other candidates are omitted so the positions of this one digit remain easy to compare.
| Box row | Left | Middle | Right |
|---|---|---|---|
| Upper | A | B | C |
| Middle | D | E | F |
| Lower | G | H | I |
Suppose an existing six elsewhere in the upper row rules out A, B and C. Another six in the middle row rules out D, E and F. A six lower down the left column rules out G, and a six lower down the right column rules out I. H is the only surviving position inside the box.
- Cross off the upper and middle box rows for this digit.
- Cross off the left and right columns within the remaining box row.
- Enter six at H, even if H previously carried several different notes.
The sixes used to exclude these cells lie outside the box. You do not need an existing six inside the box; that would mean the box's six was already placed.
Translate the sketch back into notes
H might show two, six and nine. Those extra notes do not weaken the conclusion, because the box has nowhere else to put six. After placing six, erase two and nine from H and remove six from all its peers. If an unrelated cell in H's column still carries two and nine, keep those candidates there; solving H does not reserve either digit.
A useful change to the example
Remove the six that excluded I. The box now has two possible homes, H and I. Neither can be filled from this box scan alone. You may record that six is confined to the lower box row, but you have not found a hidden single. This variation is a quick way to distinguish one remaining position from several positions that happen to sit close together.
Where to play
Ninefold includes illustrated technique lessons on iOS. Use a box scan during a campaign puzzle to look for a digit with just one home.