How to solve a hidden single in Sudoku
A hidden single is a digit that has only one possible place left in a row, column or box, even if that cell still has other candidates.
How it works
Every row, column and box must contain each digit exactly once. If a digit can only go in one cell of a unit, it has to go there, whatever else that cell could hold.
It is called hidden because the cell looks undecided: it may still list several candidates. The clue is in the unit around it, where that digit has nowhere else to go.
How to spot it
Pick a digit and a box, then check which cells are left once you block out every cell that sees that digit elsewhere. If one cell remains, that is a hidden single. Boxes are the easiest place to start.
A worked example
This is a step from a real puzzle, a moment where the next move needs a hidden single.
Struck-through digits are the ones that can be removed.
In row 2, 9 can only go in R2C8: every other cell there is blocked.
Where it fits in Rocksled Sudoku
Puzzles that need a hidden single are never rated below Easy. Each step of this kind adds 3 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.