Solving techniques
Hidden single
The hidden single is the most useful technique in Sudoku. It solves most Easy puzzles and is the base of many harder ideas.
A hidden single is a digit that can go in only one cell of a row, column or box, even if that cell also allows other digits. Because every unit must contain each digit once, the digit must go in that one cell.
How it works
Every row, column and box needs each digit from 1 to 9. If you pick a digit and all but one cell of the unit are blocked for it, the remaining cell must hold it. The cell may show several candidates, but the digit is "hidden" among them.
A step-by-step method
- Choose a digit, for example 9.
- Pick a unit, such as a box.
- Mark the cells where 9 cannot go: those in a row or column that already has a 9.
- If only one open cell remains, place the 9 there.
- Repeat for each digit and each unit.
Where to look first
- Boxes, rows and columns with many digits already placed.
- Digits that appear many times: they have the fewest places left.
- A box where two rows and two columns are blocked for the digit.
Common mistakes
- Only checking rows and columns, and forgetting the box.
- Missing a digit that blocks a cell from the other direction.
- Giving up after one pass. Every new digit can create new hidden singles.
Related ideas
The partner technique is the naked single. When singles run out, pairs come next: naked pair and hidden pair. See how to solve Sudoku for the full loop.
Frequently asked questions
What is a hidden single in Sudoku?
A digit that has only one possible cell left in a row, column or box.
How is it different from a naked single?
A naked single is a cell with one candidate. A hidden single is a digit with one possible cell.
Does the cell have to show only one candidate?
No. The cell may list several candidates. The deduction comes from the digit having nowhere else to go.