What is a Naked Single?
A naked single is the simplest and most direct deduction in Sudoku. It happens when, after eliminating numbers already present in a cell’s row, column and 3x3 box, only one candidate number remains for that cell. Because that single candidate is the only legal value, you can place it with complete certainty.
Why it matters
The naked single sudoku technique is fundamental. It’s quick, always correct, and often clears easy cells that unlock further placements. Many puzzles use naked singles repeatedly, and most serious solvers check for them before trying more advanced strategies.
How to find a naked single (step by step)
1. Look at an empty cell.
2. Gather the used numbers in the same row, the same column, and the same 3x3 box.
3. Eliminate those numbers from 1–9 to form the cell’s candidate list.
4. If only one number remains, that is a naked single — write it in and update pencil marks around it.
Because this method depends only on local elimination, you don’t need deep pattern recognition: just clear logic and careful tracking of candidates.
When to look for naked singles
- Always scan for them before attempting more complex techniques. They’re the lowest-effort deductions and can cascade into many more.
- After you place any number, immediately re-evaluate neighboring cells; a single placement often creates new naked singles in its row, column or box.
- When you pencil-mark candidates, a naked single will appear as a cell with a single tiny digit. If you don’t use pencil marks, mentally check suspicious cells (those with many filled neighbors).
Naked single vs hidden single
Don’t confuse a naked single with a hidden single. A naked single is a cell whose own elimination leaves a single candidate. A hidden single is a number that can only go in one cell of a row/column/box even though that cell may have other candidates. The naked single is stronger and easier to spot once candidates are known.
Worked example (use the visuals)
In the example candidate grid provided, focus on row 9, column 9 (notation: row 9, column 9). Every number from 1 to 8 already appears somewhere in row 9, and column 9 and the 3x3 box that contains the cell also contain the same numbers. When you list the remaining candidates for that empty cell you find only a single number is possible. That single candidate makes the cell a naked single: you can place the number immediately and update the rest of the grid.
Why a candidate grid helps
Writing out or keeping candidates in a consistent format makes naked singles obvious. With candidates present you no longer need to recompute the allowed numbers for every cell from scratch—cells with a single candidate jump out visually and speed up your solving.
Practical tips for everyday solvers
- Pencil in candidates: even rough pencil marks help you spot naked singles fast. You don’t need every candidate everywhere, just keep them in affected rows/columns/boxes.
- Scan rows, columns and boxes systematically after each placement. A quick pass of the grid returns naked singles early and prevents you from missing simple moves.
- Use elimination in order: eliminate using the row, then column, then box. That systematic approach prevents mistakes.
- Mark single-candidate cells clearly (with a darker pencil or a mental check). Writing them in immediately is usually best; leaving them as candidates can cause you to miss their effect on neighbors.
- Don’t overthink: if a cell truly has only one possible number, place it. That move requires no speculative chains or assumptions.
Common mistakes to avoid
- Forgetting to update candidates after each placement. A newly placed number can remove candidates in up to 20 other cells (its row, column and box).
- Mistaking a hidden single for a naked single and vice versa. They’re similar but rely on different reasoning.
- Relying only on intuition instead of checking candidates. Naked singles are logical; verify the elimination before writing numbers.
Practice and next steps
Start training by scanning simple puzzles for naked singles until the pattern becomes automatic. As you get comfortable, combine this habit with spotting hidden singles and pairs; many puzzles yield to a few rounds of these basic techniques before any advanced strategy is needed.
Summary
The naked single sudoku technique is a reliable, low-effort method that every solver should use consistently. It’s often the first step in solving and keeps your solving clean and error-free. Pencil candidates, scan regularly, and place any lone candidate immediately — your solving speed and accuracy will improve quickly.