When only one digit can fit in a cell, place it with confidence.
Also known as: Sole Candidate
Look for a cell whose candidates have all been eliminated but one — every other digit from 1 to 9 already sits in its row, column, or box. That surviving digit is the answer, so place it. With pencil marks on, a Naked Single is any cell showing a single mark.
A Naked Single occurs when a cell has only one possible digit remaining. Every other digit from 1 to 9 has been eliminated by the row, column, or box that cell belongs to. Since exactly one digit fits, you can place it immediately.
This is the most fundamental solving technique in Sudoku. If you can identify Naked Singles, you can solve every Easy puzzle and most Medium puzzles. Every other technique in Sudoku ultimately creates Naked Singles. They strip away candidates until only one remains.
Naked Singles appear frequently in Easy and Medium puzzles. In harder puzzles, you'll still find them, but usually only after applying more advanced elimination techniques first.
Every cell in Sudoku belongs to three units: a row, a column, and a 3×3 box. The rules state that each unit must contain the digits 1 through 9 exactly once. This means any digit already placed in a cell's row, column, or box is eliminated as a candidate for that cell.
A Naked Single forms when eight of the nine digits are eliminated. Suppose a cell is in row 3, column 5, and box 2. If the digit 1 appears in row 3, digit 2 appears in column 5, digits 3, 4, and 5 appear in box 2, digits 6 and 7 appear elsewhere in row 3, and digit 9 appears in column 5, that leaves only digit 8. The cell at R3C5 must contain 8.
The key insight is that you don't need to see all eight eliminations explicitly. If you're tracking candidates (pencil marks), a Naked Single is simply any cell with exactly one pencil mark. Without pencil marks, you scan the row, column, and box to mentally count which digits are already placed.
Consider the cell at R9C6 (row 9, column 6). Let's check what digits are already present in its three units.
Step 1: Scan row 9. The digits 1, 3, 4, 6, 7, 8, and 9 are already placed. That eliminates seven candidates.
Step 2: Scan column 6. The digits 2, 3, 4, 7, and 9 are placed in other rows. Digit 2 is the only new elimination beyond what row 9 already covered.
Step 3: Scan box 8 (bottom-center). It contains digits 1, 3, 4, 6, 7, 8, and 9. No new eliminations beyond what the row and column already provided.
Step 4: Count what's left. Digits 1, 2, 3, 4, 6, 7, 8, 9 are all eliminated. Only digit 5 remains. Place 5 in R9C6.
After placing this digit, check whether the new 5 creates Naked Singles in adjacent cells. Often, one placement triggers a chain of others. This cascade is what makes Easy puzzles flow quickly.
Scan cells at busy intersections, where a well-filled row, column and box overlap — those cells have the most digits eliminated, so a lone survivor is likeliest there. If mental scanning is hard, turn on pencil marks: any cell left showing a single mark is a Naked Single.
A Naked Single has exactly one candidate left in the cell itself, whereas a Hidden Single is a digit with only one possible cell in a row, column or box — and that cell may still show other candidates. Naked describes the cell; hidden describes a digit's options within a unit.
No, you do not need to see them explicitly. Every cell belongs to a row, a column and a 3×3 box, and between them those three units account for the digits that are ruled out. If only one digit is left unaccounted for, it is the answer. Pencil marks record the eliminations for you.
Yes, Sole Candidate is another name for the same technique: a cell in which every digit but one has been eliminated by its row, column or box. You will also see it described as a cell with a single remaining possibility. The logic and the placement are identical under either name.
Naked Singles alone are enough for every Easy puzzle and most Medium ones. Harder puzzles still contain them, but usually only after other techniques have stripped candidates away first. That is the general pattern: advanced methods eliminate candidates, and the Naked Singles they create are what actually fill the grid.
Check the cells around it straight away. The digit you placed is now eliminated from its own row, column and box, which frequently leaves a neighbouring cell with one candidate, and that placement can trigger the next. This cascade is what makes Easy puzzles flow quickly once the first few cells fall.
Explore all 28 solving techniques in our complete technique guide.