Sudoshi

Hidden Singles and Naked Singles

Almost every easy and medium Sudoku can be solved with just two techniques. Learn to spot them and you will rarely be stuck — they are also exactly what Sudoshi’s hints point out.

Candidates: what could go here?

For any empty cell, the candidates are the digits not yet used in its row, its column and its 3×3 box. Every technique below is just a clever way of narrowing those candidates down until only one remains.

Hidden single

Pick a digit and a unit — a row, a column or a box. If that digit is missing from the unit and only one empty cell in it can still take the digit, it must go there. The digit is “hidden” because the cell itself may have several candidates; it is the unit that forces it.

5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
3
1
7
2
6
6
2
8
4
1
9
5
8
7
9
Scan one box at a time: for each missing digit, rule out the cells whose row or column already has it.

Hidden singles are the most common step in any Sudoku. Start with boxes, then rows and columns: digits that already appear often on the board are the quickest to place.

Naked single

Now look at a single cell. If eight different digits already appear in its row, column and box combined, only one digit is left for it. That is a naked single: the cell has exactly one candidate.

Naked singles show up more and more as the board fills, so switch to them when scanning for hidden singles stops paying off.

A routine that works

  • Scan every box for hidden singles, digit by digit.
  • Do the same for rows and columns.
  • Check crowded cells for naked singles.
  • Each placed digit creates new chances — go round again.

Stuck? In Sudoshi, a hint never just gives away a digit: it tells you which of these techniques finds the next one, and where.

Ready to try it yourself?

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Keep reading

What is Sudoku?

The Rules of Sudoku

Killer Sudoku and Sudoku X

All guides