Hidden Single

Level 2 · White Belt (Yellow Stripe)

Strategy:

Requires:

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

Look for Hidden Single.

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

1

Hidden Single

Check the candidates for digit 3 in row 1.

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

2

Hidden Single

Digit 3 cannot go in r1c1 because r3c1 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 2: Hidden Single — Digit 3 cannot go in r1c1 because r3c1 is already in this column. Remove the entry or choose a different digit.

3

Hidden Single

r1c2 is already filled with 1.

Sudoku grid illustrating step 3: Hidden Single — r1c2 is already filled with 1.

4

Hidden Single

r1c3 is already filled with 2.

Sudoku grid illustrating step 4: Hidden Single — r1c3 is already filled with 2.

5

Hidden Single

r1c5 is already filled with 4.

Sudoku grid illustrating step 5: Hidden Single — r1c5 is already filled with 4.

6

Hidden Single

r1c6 is already filled with 8.

Sudoku grid illustrating step 6: Hidden Single — r1c6 is already filled with 8.

7

Hidden Single

r1c7 is already filled with 9.

Sudoku grid illustrating step 7: Hidden Single — r1c7 is already filled with 9.

8

Hidden Single

r1c8 is already filled with 7.

Sudoku grid illustrating step 8: Hidden Single — r1c8 is already filled with 7.

9

Hidden Single

r1c9 is already filled with 6.

Sudoku grid illustrating step 9: Hidden Single — r1c9 is already filled with 6.

10

Hidden Single

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

Sudoku grid illustrating step 10: Hidden Single — Digit 3 can only go in one cell in this row.

11

Hidden Single

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

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

12

Hidden Single

Place 3 at r1c4 - it's the only possible location in row 1.

Sudoku grid illustrating step 12: Hidden Single — Place 3 at r1c4 - it's the only possible location in row 1.

13

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.