Start Here: Solving Your First Sudoku with Naked Singles
Look at an empty cell in a Sudoku grid. Read the row it sits in, the column it sits in, and the 3×3 box around it. Cross off every digit that already appears in any of those three. If exactly one digit survives, you have found a naked single — the cell can only be that number, and you can write it in with no guessing.
That is the entire technique, and it is the one that solves most of an easy puzzle. Before you reach for anything fancier, you want to find every naked single on the board, because each one you fill in removes a candidate from its neighbours and often creates the next naked single in turn.
A worked example
Say the cell you are looking at is empty, and its row already contains 1, 2, 4, 6, and 9. Its column adds 3 and 5. Its box adds 8. That is 1, 2, 3, 4, 5, 6, 8, and 9 all spoken for. The only digit left is 7. Nothing else can legally go there, so the cell is 7 — a naked single.
Now fill it in and notice what changed. Every other empty cell in that same row, column, and box just lost 7 as an option. A cell that previously had two candidates, say 7 and 3, now has only 3. That is a new naked single, created by the move you just made. This chain reaction is why naked singles are worth hunting systematically rather than at random.
Where to look first
You will find naked singles fastest in the most crowded parts of the grid, because crowded units leave fewer survivors. A practical scanning order:
- Start with the row, column, or box that already has the most digits filled in — it has the fewest candidates left, so a naked single is most likely there.
- After every digit you place, re-check the row, column, and box it belongs to. The move you just made is the most likely place a new single appeared.
- When a digit is nearly complete across the grid (say seven of the nine 5s are placed), focus on the units still missing it — those empty cells are heavily constrained.
Let pencil marks do the bookkeeping
Holding eight crossed-off digits in your head for every cell is tiring, and it is where mistakes creep in. Pencil marks fix that: in each empty cell you note the digits still possible. A naked single is then visually obvious — it is any cell with a single pencil mark left.
In Sukuro you can turn on automatic pencil marks from the controls, and on the early learning levels each digit also carries its own colour, so a value that repeats in a row or box jumps out before you have even finished scanning. The teaching levels exist precisely to make this pattern-spotting automatic, so that by the time the assists switch off you are doing it by eye.
When the naked singles run out
On an easy puzzle, naked singles alone will carry you to the finish. On harder grids you will reach a point where no cell has only one candidate left — and that is not a dead end, it is your cue to switch techniques.
The next one to learn is the hidden single: a digit that can only go in one cell of a row, column, or box, even though that cell still shows several candidates. Where a naked single asks "what can go in this cell?", a hidden single asks "where can this digit go in this unit?" — the same logic viewed from the other direction. Master both and you can solve the large majority of Sudoku you will meet.
Practice it now
The fastest way to internalise naked singles is to play a few easy boards and consciously narrate the logic — "row has these, column has these, only this digit is left" — until you stop needing to. Sukuro's learning levels 1–10 turn the assists on for exactly this, and the Hint button will show you the next logical step and the reason behind it whenever you get stuck.