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    Naked Single Technique

    Beginner2025-01-17• By Sudoku Master Team• 8 min read
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    Quick Summary

    A Naked Single is a cell that can only contain one possible digit after eliminating all numbers already present in its row, column, and box. This is the most fundamental Sudoku solving technique.

    What is a Naked Single?

    A Naked Single is a cell in Sudoku that, after eliminating all numbers already present in its row, column, and box, has only one possible candidate left. This is a guaranteed logical move—no guessing required. It's often the first technique beginners learn and remains useful throughout all difficulty levels.

    Why is it Called "Naked Single"?

    The term "naked" refers to the fact that the single candidate is clearly visible without needing to look at other cells or apply complex logic. The single digit is "naked" or exposed, making it obvious to spot. This distinguishes it from "hidden singles" where the single candidate might be hidden among other possibilities in the same cell.

    How to Identify a Naked Single

    Follow these steps to find naked singles:

    1. 1Choose an empty cell in the puzzle
    2. 2List all possible digits (1-9) for that cell
    3. 3Eliminate digits that already appear in the same row
    4. 4Eliminate digits that already appear in the same column
    5. 5Eliminate digits that already appear in the same 3×3 box
    6. 6Check the result: If only one digit remains, you've found a naked single!

    Step-by-Step Example

    Let's work through a detailed example. In the puzzle below, we'll focus on the cell at row 2, column 1 (highlighted in yellow). Let's see why this is a naked single:

    Naked Single Example - Target Cell9 by 9 Sudoku grid with numbers. Thick lines separate 3x3 boxes.53719598686348317266284195879
    The highlighted cell (row 2, column 1) is our target. Let's analyze what digit can go here.

    Step 1: Check the Row (Row 2)

    Row 2 currently contains: 1, 9, 5

    We can eliminate: 1, 9, 5

    Step 2: Check the Column (Column 1)

    Column 1 currently contains: 5, 6, 8, 4, 7

    We can eliminate: 5, 6, 8, 4, 7

    Step 3: Check the Box (Top-left 3×3 box)

    The top-left box currently contains: 5, 3, 9, 8

    We can eliminate: 5, 3, 9, 8

    Step 4: Determine the Result

    Starting with all digits 1-9, after elimination we have:

    Original possibilities: 1, 2, 3, 4, 5, 6, 7, 8, 9

    Eliminated from row: 1, 5, 9

    Eliminated from column: 4, 5, 6, 7, 8

    Eliminated from box: 3, 5, 8, 9

    Remaining possibilities: 2

    Result: Only the digit 2 remains possible for this cell. This is a classic naked single!

    Naked Single Solved9 by 9 Sudoku grid with numbers. Thick lines separate 3x3 boxes.537219598686348317266284195879
    The cell now contains 2, which was the only possible digit for this position.

    Practice Exercise

    Try finding naked singles in this practice puzzle:

    Practice Puzzle9 by 9 Sudoku grid with numbers. Thick lines separate 3x3 boxes.37619598686348317266284195879
    Find all the naked singles in this puzzle. Look for cells that can only contain one digit.

    Common Patterns and Tips

    1. Look for Rows/Columns/Boxes with Many Filled Cells

    Naked singles often appear in rows, columns, or boxes that are nearly complete. If a unit has 7 or 8 cells filled, the remaining empty cells are good candidates for naked singles.

    2. Check Intersections

    Pay special attention to cells where a row and column with many filled cells intersect, especially if they're also in a box with many filled cells.

    3. Use Pencil Marks

    While not always necessary for naked singles, keeping track of possible digits in each cell can help you spot them more quickly and avoid mistakes.

    4. Scan Systematically

    Don't just look randomly. Go through each row, column, or box systematically to ensure you don't miss any naked singles.

    Advanced Applications

    While naked singles are basic, they're essential for more advanced techniques. Many complex patterns become apparent only after naked singles have been found and filled in. They also help create new naked singles in other parts of the puzzle.

    Common Mistakes to Avoid

    • •Incomplete elimination: Make sure you check all three constraints (row, column, box)
    • •Overlooking the obvious: Sometimes the most obvious naked singles are missed
    • •Rushing: Take time to verify each elimination step
    • •Forgetting to update: After placing a number, check if it creates new naked singles

    When to Use This Technique

    Naked singles should be your first technique when starting any Sudoku puzzle. They're most common in:

    • •Easy and medium difficulty puzzles
    • •Puzzles with many given numbers
    • •After applying other techniques that create new constraints
    • •As a stepping stone to more advanced techniques

    Next Steps

    Once you've mastered naked singles, you're ready to learn about Hidden Singles, which are slightly more complex but equally important. Hidden singles occur when a digit can only go in one cell within a row, column, or box, even though that cell might have other possible digits.

    Ready for More?

    Practice your naked single skills with our interactive puzzles, or learn the next technique.