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 the unit-level question:
Where else could this digit go?
If the answer is “nowhere,” the digit is forced into that cell.
How a Hidden Single works
Each row, column, and box must contain every digit 1–9 exactly once in a completed classic Sudoku.
Choose a digit that is missing from a unit — say 6 in row 3.
Now inspect every empty cell in that row:
- some cells cannot contain 6 because their columns already contain 6;
- others cannot contain 6 because their boxes already contain 6;
- if only one cell remains possible, 6 must be placed there.
The target cell does not need to be reduced to {6}. It only needs to be the only location for 6 in that unit.
Step-by-step Hidden Single example
Hidden Single in a box
Boxes are a common place to find Hidden Singles visually.
Choose a digit missing from the box and use crossing rows and columns to block cells. If only one cell remains, the digit is forced.
This search method is usually called crosshatching.
The logic is still Hidden Single logic: the digit has one possible position in the box.
Hidden Single in a column
The same rule works vertically.
If digit 4 is missing from column 7 and every unsolved cell except r5c7 is blocked from containing 4, then r5c7 = 4.
Rows, columns, and boxes are equal units under the classic Sudoku constraints.
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.
Hidden Single
Where can this digit go in this unit?
Only one cell.
A Hidden Single may look like:
{2, 6, 9}
while a Naked Single looks like:
{6}
Both produce a forced placement, but the reasoning uses different information.
How to find Hidden Singles without full candidates
You can use digit scanning.
- Choose a digit.
- Find a row, column, or box that does not contain it.
- Use existing copies of that digit in crossing units to rule out positions.
- If only one cell remains, place the digit.
This is often faster on paper than writing all candidates in every cell.
How to find Hidden Singles with candidates
If full notes are visible, scan one unit at a time and count occurrences of a chosen candidate.
For example, suppose row 8 contains:
r8c1 {1, 3}r8c4 {1, 5, 7}r8c6 {2, 5}r8c9 {1, 2, 8}
Candidate 7 appears only in r8c4, so r8c4 = 7.
The other candidates in r8c4 can be removed when 7 is placed.
Why Hidden Singles are easy to overlook
With notes visible, your eyes naturally notice cells with very few candidates. Hidden Singles can sit inside much busier cells.
The fix is to stop counting candidates in the cell and scan one digit across the unit.
A candidate filter can help during learning: show only one digit across the grid, then inspect which units contain exactly one occurrence of that candidate.
Hidden Singles after eliminations
Intermediate and advanced techniques often exist to create Hidden Singles.
For example:
- Locked Candidates removes a 5 from two cells.
- In column 4, only one candidate 5 remains.
- That cell becomes a Hidden Single for 5.
This is why you should return to Singles after every elimination.
Common Hidden Single mistakes
Checking only the target cell
Hidden Single is unit logic. You must compare the candidate's possible positions across the entire row, column, or box.
Assuming the cell must have one candidate
That would be a Naked Single. A Hidden Single cell may have several notes.
Forgetting one candidate position
If the digit can legally appear in two cells of the unit, it is not a Hidden Single yet.
Looking for all digits at once
Choose one digit and one unit. A focused scan reduces visual noise.
Not rescanning after eliminations
One removed candidate can create a new Hidden Single immediately.
FAQ
Why is it called a Hidden Single?
Because the forced digit is usually mixed with other candidates inside its cell. It becomes a Single only when you compare that digit's positions across the unit.
Can a Hidden Single also be a Naked Single?
A placement may sometimes satisfy more than one simple description, but the teaching distinction is based on the reasoning used. If the cell has one candidate, Naked Single is sufficient; if the digit has one location while the cell has other candidates, it is clearly Hidden Single logic.
Where should I look for Hidden Singles first?
Boxes can be convenient for crosshatching, while nearly complete rows and columns are also strong targets. With full notes, scan each candidate digit across a unit.
Do Hidden Singles require pencil marks?
No. They can be found by crosshatching and visual digit scanning. Notes simply make candidate positions explicit.
What to learn next
Once Naked and Hidden Singles are reliable, practice them together in Beginner Practice. Then move to Locked Candidates, where a digit may be restricted to a line inside a box without yet having one exact cell.