Proven Sudoku
Hidden Single
Also called Unique Candidate, Pinned Digit, Only Square, Single Position.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
Look at one digit in one row, column or box. If there is only one square left where it can go, put it there. The square may still list other digits, but they cannot be right.
The exact rule, for stronger players
The rule, stated exactly: For a house H and digit d, if exactly one cell of H has d as a candidate, that cell takes d.
What it removes: Place d in the sole candidate cell, then remove every other candidate from that cell and remove d from all peers.
Why it works: Every digit appears exactly once in every house. If d can only be placed in one cell of H, that placement is forced regardless of what else the cell might have contained.
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.
Learn these first
- Naked SingleA square has only one digit left that fits.
Related singles
- Full HouseA row, column or box has just one empty square left.
How often does it come up?
Our rough estimate: universal, and the single highest-value deduction in real solving. Cross-hatching finds these without any pencil marks.