Hidden Single

A Hidden Single occurs when a digit can only go in one cell within a row, column, or box — even if that cell has other candidates. The digit is "hidden" amongst the other possibilities but is the only place it can go in that unit.

How It Works

While a Naked Single looks at a single cell and finds only one candidate, a Hidden Single looks at a single digit and finds only one cell where it can go within a unit (row, column, or box).

Example

Look at Row 1 and ask: where can 6 go?

R1C1: 2 (given)
R1C2: candidates 4, 5, 9 — no 6
R1C3: candidates 1, 5, 6, 9 — has 6!
R1C4-7: candidates don't include 6
R1C8-9: 3, 8 (given)

R1C3 is the only cell in Row 1 where 6 can go. Even though R1C3 has four candidates, it must be 6 — that's a Hidden Single.

puzzle: S9B028A8Z8307838303084QBU83BT830685071703DE83BP04BP06850Z228E088Z028B078313018G88DE82DMBSBO061U8407CJ03BN04BP0Z2Q2W042X082X131V096A06140483A14N4J2V0901122U062U4M4Q02
mode: guided
technique: Hidden Single
initial:
  layers:
    hints: true
steps:
  - text: >
      Where can 6 go in Row 1? Let's use Focus Mode to highlight all 6s.
    hint: subtle
    technique: HS
    state:
      selection:
        cell: R1C3
  - text: >
      R1C3 is the only cell in Row 1 with candidate 6.
    hint: obvious
    technique: HS
    state:
      selection:
        cell: R1C3
      focus:
        enabled: true
        digits: [6]
  - text: >
      Since 6 can only go in R1C3 within Row 1, that cell must be 6.
    hint: detailed
    technique: HS

When to Use It

Hidden Singles are slightly harder to spot than Naked Singles because you need to think about each digit's possibilities across a whole unit. They're most effective when:

Many cells in a unit are already filled
You're systematically checking where each digit can go
Focus Mode is highlighting a specific digit

Tips

Use Focus Mode to highlight all cells containing a specific digit
Check each box, row, and column for digits that appear in only one cell
After finding a Hidden Single, check for Naked Singles it may create

The Difference

Technique	Perspective	Question Asked
Naked Single	Cell-focused	"What can go in this cell?"
Hidden Single	Digit-focused	"Where can this digit go?"

More Puzzles

Hidden Single ex. 1

Related Techniques

Naked Single — When a cell has only one candidate