Naked Single: Step-by-Step Guide
A Naked Single is a cell with exactly one possible digit. When you eliminate every digit already present in a cell's row, column, and 3×3 box, if only one digit remains, it must go there. No guessing, no logic chains — just forced placement. This is the foundation of Sudoku solving.
Prerequisites
None. Naked Single is the first technique to learn. Start here.
How to find a Naked Single
- Pick any empty cell.
- Note every digit already placed in the same row (9 cells), column (9 cells), and 3×3 box (9 cells).
- Cross those digits off the list 1–9.
- If exactly one digit remains, that's a Naked Single — write it in.
Worked example
Consider an empty cell (marked ?) where:
- Its row contains: 1, 2, 4, 5, 6, 8, 9
- Its column contains: 3, 7
- Its box contributes no new digits
Combined eliminations: 1, 2, 3, 4, 5, 6, 7, 8, 9 — all but 3 eliminated by the row and all but 3 and 7... wait, let's be precise:
Digits 1–9: 1 2 3 4 5 6 7 8 9
From row: ✕ ✕ ✕ ✕ ✕ ✕ ✕
From column: ✕ ✕
Remaining: 3
Only 3 survives. Place 3 in the cell. That's a Naked Single.
Why it works
Sudoku's rules require every digit 1–9 to appear exactly once in each row, column, and 3×3 box. If eight of the nine digits are already placed in the units that govern a cell, the ninth digit is the only legal value. Placing it is not a guess — it is a deduction from the rules.
Common mistakes
- Checking only the row. You must eliminate from the row, column, and box. Missing one unit leaves phantom candidates.
- Forgetting to update candidates. After placing a digit, remove it from every peer cell in that row, column, and box. New Naked Singles often appear immediately.
- Confusing Naked Single with a cell that just looks empty. A single empty cell in a row is a Hidden Single, not necessarily a Naked Single — verify by eliminating from all three units.
Frequently asked questions
What is a Naked Single in Sudoku?
A Naked Single is a cell where only one digit is possible after eliminating all digits in its row, column, and 3×3 box. Place that digit immediately — it's logically forced.
What is the difference between a Naked Single and a Hidden Single?
A Naked Single: only one candidate in a specific cell. A Hidden Single: a digit that can go in only one cell within a whole row, column, or box — the cell may appear to have multiple candidates but only one valid digit remains when you look at the whole unit.
How do you find Naked Singles quickly?
Use pencil marks. Write small candidate digits in each empty cell as you work. Any cell with a single pencil mark is a Naked Single. Good Sudoku software highlights them automatically. With practice, you can spot near-naked cells (2–3 candidates) by eye.
Is Naked Single enough to solve any Sudoku?
No. Easy puzzles usually yield to Naked Singles and Hidden Singles alone. Medium puzzles often require Locked Candidates. Hard and Expert puzzles need intermediate and advanced techniques like X-Wing, Swordfish, and beyond. But every puzzle starts with scanning for Naked Singles.
What should I learn after Naked Single?
Learn Hidden Single next (a digit that fits in only one cell in a unit), then Locked Candidates (box-line interactions), and then X-Wing for hard puzzles.
Next technique: Locked Candidates · All Techniques