Difficulty tier T1
Hidden Singles in Sudoku: The Fastest Move
If you learn to see one thing instantly, make it the hidden single. It's the most common placement in fast solving and the reason strong solvers barely use pencil marks on easy and medium boards.
What a hidden single is
A hidden single is a digit that can legally go in only one cell of a box, row, or column — even if that cell also shows other candidates. Every other cell in the unit is blocked: the digit already sits in their row, column, or box. So the one surviving cell is forced.
It's called hidden because the cell often looks busy with candidates, so the placement isn't obvious the way a naked single is. You find it by thinking about the digit, not the cell.
The scan that finds them
The fastest solvers don't read every cell's candidates. They scan by digit:
- Pick a digit, 1 through 9.
- Look at one box and ask: where can this digit still legally go?
- If exactly one cell survives, place it.
- Move to the next box, then the next digit.
Do this on a fixed route and most of a puzzle falls without a single pencil mark. That's the core habit behind getting faster at sudoku: make the cheapest move — the hidden single — automatic.
Why this is the T1 workhorse: in Speedoku's difficulty model, hidden singles sit at tier T1 alongside naked and open singles — the moves you make constantly. A board that never needs anything harder is an "easy" board by definition. Speed here is worth more than any advanced technique.
A quick habit that compounds
Every time you place a digit, re-scan its row, column, and box before moving on. A placement is exactly when new hidden singles appear — usually the next move is right next to the last one. Chaining placements this way is what turns a slow, hunting solve into a smooth one.
Put it to the test on today's Daily Challenge — everyone gets the same board — or practice free as long as you like.
Frequently asked questions
What is a hidden single in sudoku?
- A hidden single is a digit that can legally go in only one cell within a box, row, or column, even if that cell shows several candidates. Every other cell in the unit is blocked from taking that digit by a clash in its row, column, or box, so the one remaining cell must take it.
What's the difference between a hidden single and a naked single?
- A naked single is a cell with only one candidate left — you look at the cell. A hidden single is a digit with only one home in a unit, even though its cell may show other candidates — you look at the digit across the unit. Hidden singles are faster to spot by eye, which is why fast solvers lead with them.
How do I find hidden singles quickly?
- Scan digit by digit. Take a digit, look at a box, and ask where in that box it can still legally go. If only one cell survives, place it. Sweeping 1 through 9 across the boxes finds the large majority of your placements without writing any pencil marks.