Proven Sudoku
Naked Single
Also called Sole Candidate, Forced Digit, Singleton, Lone Single.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
A square has only one digit left that fits. Every other digit already appears in its row, its column or its box. So that square is settled.
The exact rule, for stronger players
The rule, stated exactly: For a cell c, if the candidate set of c has cardinality one, the sole remaining candidate is the digit of c.
What it removes: Place the sole candidate in the cell, then remove that digit from the candidates of all 20 peers.
Why it works: A cell's candidate set is the set of digits not eliminated by its peers. Every cell must hold exactly one digit. If elimination has reduced the set to a single member, that member is forced.
Worked examples
These come from real Proven Sudoku boards. Nobody drew them by hand to make the pattern look neat. Each one is checked first, so the board is one you could really meet, the pattern is really there, and every digit it rules out is really wrong.
Related singles
- Full HouseA row, column or box has just one empty square left.
- Hidden SingleLook at one digit in one row, column or box.
How often does it come up?
Our rough estimate: universal. Present in every puzzle at every difficulty, and the dominant deduction in easy grids.