Hidden Single

Level 2 · White Belt (Yellow Stripe)

Strategy:

Learn after:

Hidden Pair Locked Candidates

Overview

A Hidden Single occurs when a digit can only go in one cell within a house (row, column, or block), even though that cell may have other candidates. The digit is "hidden" among other possibilities but is forced by the constraint of its house.

How It Works

For each house (row, column, or block), examine each digit 1-9.
Count how many cells in that house can contain each digit.
If a digit can only go in one cell within that house, place it there.
The cell may have multiple candidates, but within the house, only one cell can hold that specific digit.

Example

Starting position

Look for Hidden Single.

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

1

Scan row 1 for 3

Check the candidates for digit 3 in row 1.

Sudoku grid illustrating step 1: Scan row 1 for 3 — Check the candidates for digit 3 in row 1.

2

3 is blocked from r1c1 by the box

Digit 3 cannot go in r1c1 because r2c3 is already in this box. Remove the entry or choose a different digit.

Sudoku grid illustrating step 2: 3 is blocked from r1c1 by the box — Digit 3 cannot go in r1c1 because r2c3 is already in this box. Remove the entry or choose a different digit.

3

3 is blocked from r1c2 by the box

Digit 3 cannot go in r1c2 because r2c3 is already in this box. Remove the entry or choose a different digit.

Sudoku grid illustrating step 3: 3 is blocked from r1c2 by the box — Digit 3 cannot go in r1c2 because r2c3 is already in this box. Remove the entry or choose a different digit.

4

r1c3 is already filled

r1c3 is already filled with 4.

Sudoku grid illustrating step 4: r1c3 is already filled — r1c3 is already filled with 4.

5

r1c4 is already filled

r1c4 is already filled with 2.

Sudoku grid illustrating step 5: r1c4 is already filled — r1c4 is already filled with 2.

6

3 is blocked from r1c5 by the box

Digit 3 cannot go in r1c5 because r3c4 is already in this box. Remove the entry or choose a different digit.

Sudoku grid illustrating step 6: 3 is blocked from r1c5 by the box — Digit 3 cannot go in r1c5 because r3c4 is already in this box. Remove the entry or choose a different digit.

7

3 is blocked from r1c6 by the box

Digit 3 cannot go in r1c6 because r3c4 is already in this box. Remove the entry or choose a different digit.

Sudoku grid illustrating step 7: 3 is blocked from r1c6 by the box — Digit 3 cannot go in r1c6 because r3c4 is already in this box. Remove the entry or choose a different digit.

8

r1c7 is already filled

r1c7 is already filled with 8.

Sudoku grid illustrating step 8: r1c7 is already filled — r1c7 is already filled with 8.

9

3 is blocked from r1c9 by the column

Digit 3 cannot go in r1c9 because r5c9 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 9: 3 is blocked from r1c9 by the column — Digit 3 cannot go in r1c9 because r5c9 is already in this column. Remove the entry or choose a different digit.

10

Only one cell for 3 in the row

Digit 3 can only go in one cell in this row.

Sudoku grid illustrating step 10: Only one cell for 3 in the row — Digit 3 can only go in one cell in this row.

11

Hidden Single in the row

This is a Hidden Single in the row: the digit has only one possible cell.

Sudoku grid illustrating step 11: Hidden Single in the row — This is a Hidden Single in the row: the digit has only one possible cell.

12

Place 3 at r1c8

Place 3 at r1c8 - it's the only possible location in row 1.

Sudoku grid illustrating step 12: Place 3 at r1c8 — Place 3 at r1c8 - it's the only possible location in row 1.

13

Result

Result after applying the technique.

Sudoku grid showing the result after applying the technique

Tips & Patterns

Blocks often reveal Hidden Singles more easily than rows or columns.
Focus on digits that appear frequently elsewhere in the puzzle.
Hidden Singles become more apparent after eliminations from other techniques.