Hidden Single
A Hidden Single appears when a digit can go in only one position within a row, column, or box. Even if the cell shows multiple candidates, the placement is logically forced.
Coordinates are written as E5 style (row letter + column number). For compatibility, we
occasionally show the equivalent R5C5 notation.
When to use it
- After a full Naked Single pass produces no placements.
- When candidate notes are dense and difficult to compare cell by cell.
- As part of a fixed scan loop by row, then column, then box.
Steps
- Choose one unit (row, column, or 3x3 box).
- Track where each digit 1-9 can appear in that unit.
- If one digit has exactly one valid position, place it.
Digit-first scanning method
- Scan one digit at a time, not one cell at a time.
- Keep a fixed pass order: rows A-I, columns 1-9, then boxes.
- After confirming one Hidden Single, re-check the same digit nearby first.
Concrete example (valid)
In row R5, suppose digit 7 can be placed at C2, C5, C8. If column and box constraints remove C2 and C8, then R5C5 = 7 is forced.
7
1
7
9
7
2
7
7
Keep Remove Focus Given Rows A-I / Columns 1-9
Digit 7 can be placed in exactly one location in row R5, so R5C5 is forced.
Invalid case (do not place)
If digit 7 still has two positions in the same unit (for example C2 and C5), Hidden Single does not apply.
7
1
7
9
7
2
4
Keep Remove Focus Given Rows A-I / Columns 1-9
Two valid positions remain for the same digit, so no Hidden Single yet.
Common mistakes
- Scanning from a cell perspective only and skipping digit perspective.
- Changing scan order every pass, which increases misses.
- Not rescanning related units after each confirmed placement.