Naked Single

Level 2 · White Belt (Yellow Stripe)

Strategy:

Learn after:

Overview

A Naked Single is the simplest pencilmark technique. When a cell has only one candidate remaining (after all eliminations from its row, column, and block), that candidate must be the solution.

How It Works

Enable pencilmarks to see all candidates for each cell.
Look for any cell that has only one candidate remaining.
That candidate is the solution for the cell.

Example

Starting position

Look for Naked Single.

Sudoku grid showing the initial puzzle state for Look for Naked Single.

1

Check the candidates in r4c4

Check cell r4c4 and examine its candidates.

Sudoku grid illustrating step 1: Check the candidates in r4c4 — Check cell r4c4 and examine its candidates.

2

1 is blocked from r4c4 by row and column

Digit 1 cannot go in r4c4 because r4c1 is in this row and r5c4 is in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 2: 1 is blocked from r4c4 by row and column — Digit 1 cannot go in r4c4 because r4c1 is in this row and r5c4 is in this column. Remove the entry or choose a different digit.

3

2 is blocked from r4c4 by the row

Digit 2 cannot go in r4c4 because r4c9 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 3: 2 is blocked from r4c4 by the row — Digit 2 cannot go in r4c4 because r4c9 is already in this row. Remove the entry or choose a different digit.

4

3 is blocked from r4c4 by the box

Digit 3 cannot go in r4c4 because r6c5 is already in this box. Remove the entry or choose a different digit.

Sudoku grid illustrating step 4: 3 is blocked from r4c4 by the box — Digit 3 cannot go in r4c4 because r6c5 is already in this box. Remove the entry or choose a different digit.

5

4 is blocked from r4c4 by the row

Digit 4 cannot go in r4c4 because r4c5 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 5: 4 is blocked from r4c4 by the row — Digit 4 cannot go in r4c4 because r4c5 is already in this row. Remove the entry or choose a different digit.

6

5 is blocked from r4c4 by the row

Digit 5 cannot go in r4c4 because r4c8 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 6: 5 is blocked from r4c4 by the row — Digit 5 cannot go in r4c4 because r4c8 is already in this row. Remove the entry or choose a different digit.

7

6 is blocked from r4c4 by the row

Digit 6 cannot go in r4c4 because r4c6 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 7: 6 is blocked from r4c4 by the row — Digit 6 cannot go in r4c4 because r4c6 is already in this row. Remove the entry or choose a different digit.

8

8 is blocked from r4c4 by column and box

Digit 8 cannot go in r4c4 because r8c4 is in this column and r5c6 is in this box. Remove the entry or choose a different digit.

Sudoku grid illustrating step 8: 8 is blocked from r4c4 by column and box — Digit 8 cannot go in r4c4 because r8c4 is in this column and r5c6 is in this box. Remove the entry or choose a different digit.

9

9 is blocked from r4c4 by the column

Digit 9 cannot go in r4c4 because r6c4 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 9: 9 is blocked from r4c4 by the column — Digit 9 cannot go in r4c4 because r6c4 is already in this column. Remove the entry or choose a different digit.

10

Only 7 remains

This cell has only one candidate remaining: 7.

Sudoku grid illustrating step 10: Only 7 remains — This cell has only one candidate remaining: 7.

11

Naked Single in this cell

This is a Naked Single: the cell has only one remaining candidate.

Sudoku grid illustrating step 11: Naked Single in this cell — This is a Naked Single: the cell has only one remaining candidate.

12

Place 7 at r4c4

Place 7 at r4c4 - it's the only possibility for this cell.

Sudoku grid illustrating step 12: Place 7 at r4c4 — Place 7 at r4c4 - it's the only possibility for this cell.

13

Result

Result after applying the technique.

Sudoku grid showing the result after applying the technique

Tips & Patterns

Pencilmarks are essential for spotting Naked Singles.
After placing any digit, check its row, column, and block for new Naked Singles.
In easy puzzles, Naked Singles can chain together quickly.