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A Hidden Single is a digit with exactly one legal position inside a row, column, or box.
The target cell may still show several candidates. What matters is not how many candidates the cell has, but how many positions the target digit has in the house.
The proof is digit-first:
Where else could this digit go in this row, column, or box?
If the answer is “nowhere,” the digit is forced into the remaining cell.
- Row
- Target cell
- Candidates: 6
How a Hidden Single works
Each row, column, and box in a completed classic Sudoku must contain every digit 1–9 exactly once.
Choose one digit that is missing from a house — for example 6 in row 3.
Now inspect every unsolved cell in that row.
A position for 6 may be impossible because:
- its column already contains 6;
- its box already contains 6;
- the cell is already solved with another digit.
If all possible positions except one are blocked, 6 must occupy the surviving cell.
The target cell does not need to have candidate set {6}. It only needs to be the only location for 6 in that house.
Step-by-step Hidden Single example
Hidden Single in a row
Suppose row 8 still needs digits 2, 5, 7, and 9.
The candidate sets are:
r8c1 {2, 5}r8c4 {2, 7, 9}r8c6 {5, 9}r8c9 {2, 5, 9}
Candidate 7 appears only in r8c4.
Therefore:
r8c4 = 7
Even though r8c4 had three candidates before the placement, 7 is forced because it has one position in the row.
Hidden Single in a box
Boxes are one of the easiest places to find Hidden Singles visually.
Choose a digit missing from the box and use solved copies of that digit in crossing rows and columns to block positions.
If one cell remains possible, the digit is forced there.
This search method is commonly called crosshatching.
The logical result is still a Hidden Single: one digit has one legal position in the box.
Hidden Single in a column
The same proof works vertically.
If digit 4 is missing from column 7 and every unsolved position except r5c7 is blocked from containing 4, then:
r5c7 = 4
Rows, columns, and boxes are equally valid houses for Hidden Single logic.
The shape on the screen changes; the proof does not.
Hidden Single vs Naked Single
The easiest way to remember the difference is to change the question.
Naked Single
What can go in this cell?
Only one digit.
A Naked Single might look like:
{6}
Hidden Single
Where can this digit go in this house?
Only one cell.
A Hidden Single cell might still look like:
{2, 6, 9}
Both produce forced placements, but the information used in the proof is different.
Hidden Single vs Full House / last missing digit
A Full House is even more direct.
If a row, column, or box has only one empty cell, the missing digit must occupy that cell.
That proof starts from the number of empty cells in the house.
A Hidden Single can occur while several cells in the house remain unsolved. The target digit is forced because only one of those cells can legally contain it.
A particular placement can sometimes have more than one valid explanation. The useful distinction is the proof you actually used to find it.
How to find Hidden Singles without full candidates
Use digit-first scanning.
- Choose one digit.
- Find a row, column, or box that does not yet contain it.
- Use existing copies of that digit in crossing houses to block positions.
- Ignore cells already solved with other digits.
- If one legal position remains, place the digit.
For boxes, this becomes crosshatching.
For rows and columns, the same idea works by checking where the target digit is blocked by the perpendicular house and the relevant box.
This approach is often faster on paper than writing complete candidates early in the puzzle.
How to find Hidden Singles with candidates
With full candidates visible, scan one house and one candidate digit at a time.
Suppose a row contains candidate 7 in only one unsolved cell, even though that cell also contains other notes.
That single occurrence means 7 is forced there.
A practical candidate-based routine is:
- choose a row, column, or box;
- choose one missing digit;
- count where that candidate appears in the house;
- if the count is one, place it;
- update candidates and re-scan.
This is digit-frequency logic, not cell candidate-count logic.
Why Hidden Singles are easy to overlook
With candidate notes visible, your eyes naturally notice cells containing only two or three candidates.
A Hidden Single can be buried inside a visually busy cell with four or five notes.
The fix is to stop asking:
Which cell has few candidates?
and instead ask:
Where does digit 5 appear in this row?
or:
How many candidate 8s remain in this box?
A candidate filter can make this viewpoint much easier by showing one digit across the grid.
Hidden Singles after candidate eliminations
Hidden Singles appear throughout the entire solving process, not only at the beginning.
For example:
- Locked Candidates removes 5 from two cells in column 4;
- only one candidate 5 remains in that column;
- the surviving position becomes a Hidden Single;
- placing 5 changes its row, column, and box;
- new Singles may appear immediately.
The same thing can happen after subsets, Fish, Wings, Coloring, or Chains.
This is why the correct response to an advanced elimination is usually to return to Singles before searching for another advanced technique.
Hidden Singles and candidate filters
Digital candidate filters can make Hidden Singles much easier to recognise.
If the interface highlights all candidate 6s, scan each house:
- one 6 in a house → Hidden Single;
- two 6s in a house → possible Strong Link later;
- several 6s → no Single yet.
This builds a useful bridge from beginner logic to advanced single-digit reasoning. The same habit of following one candidate through houses later supports Locked Candidates, Fish, Coloring, and Chains.
Common Hidden Single mistakes
Checking only the target cell
Hidden Single is house-level logic. You must compare the target digit's possible positions across the entire row, column, or box.
Assuming the target cell must have one candidate
That would be a Naked Single. A Hidden Single cell may have several legal candidates before the forced digit is placed.
Forgetting one legal position
If the digit can still appear in two cells of the house, it is not a Hidden Single.
Looking for all digits at once
Choose one target digit. A focused digit scan reduces visual noise and makes omissions less likely.
Trusting incomplete pencil marks
If you use selective notes, absence of a written candidate does not necessarily mean the candidate is logically impossible. Hidden Single counting is reliable only when the relevant candidate information is complete.
Not re-scanning after eliminations
One removed candidate can create a Hidden Single instantly.
FAQ
Why is it called a Hidden Single?
Because the forced digit can be hidden among several candidates in its cell. It becomes a Single only when you compare that digit's possible positions across the house.
Can a Hidden Single also be a Naked Single?
The same placement can sometimes satisfy both descriptions. If the target cell has one candidate, Naked Single is already a sufficient proof; if the cell has several candidates but the digit has one position in the house, the reasoning is clearly Hidden Single logic.
Where should I look for Hidden Singles first?
Boxes are convenient for crosshatching, while nearly complete rows and columns are also strong targets. With full candidates, scan one candidate digit through each house.
Do Hidden Singles require pencil marks?
No. They can be found visually by digit scanning and crosshatching. Complete candidate notes simply make the possible positions explicit.
Can a Hidden Single appear in a hard Sudoku?
Yes. Advanced eliminations frequently create new Hidden Singles, so the technique remains relevant at every difficulty level.
What to learn next
Once Naked and Hidden Singles are reliable, move to Locked Candidates. There the digit is not reduced to one exact cell, but its positions become restricted strongly enough to eliminate the same digit elsewhere.