Overview
- Tier 1
- Singles
- Typical difficulty
- Peak Score 10–25
- Also known as
- Unique Candidate, Penciled Single, Pinned Digit
A Hidden Single occurs when a digit can only legally appear in one single cell within a row, column, or 3×3 box, even if that cell currently contains other candidate numbers.
Hidden Singles are the workhorse technique of human Sudoku solving. They are described as 'hidden' because the cell in question may look cluttered with multiple pencil marks, disguising the fact that for one specific digit, no other cell in that house can accept it. Most newspaper and easy-to-medium puzzles are solved through systematic cross-hatching to locate hidden singles in boxes, rows, and columns.
Worked examples
1 / 5
Reading the exampleCells forming the pattern
158
13678
9
4
18
2
378
15678
13567
1258
138
4
7
18
6
38
1589
135
18
1678
67
5
9
3
78
4
2
6
2
1
9
7
4
5
3
8
9
5
3
8
2
1
4
67
67
7
4
8
3
6
5
1
2
9
3
9
27
6
5
8
27
17
147
1458
168
56
2
34
7
9
158
1345
2458
78
257
1
34
9
6
578
3457
In column 8, the digit 9 can only go in R2C8. Every other empty cell in column 8 already sees a 9 in its row or box, so R2C8 must be 9.
Places 9 in R2C8
How to apply it
Spotting the pattern and making the deduction, in order.
- Choose a row, column, or 3×3 box and pick a digit you are tracking.
- Scan every empty cell in that unit to see if intersecting lines already block that digit from entering those cells.
- Cross out all blocked cells from consideration.
- If only one cell in the unit can legally take the digit, place the number there and clear any remaining pencil marks from the cell.
Common mistakes
- Only checking 3×3 boxes and neglecting rows and columns. Hidden singles in long lines (rows/columns) are much easier to miss than in 3×3 boxes.
- Failing to erase other pencil marks in the cell after placing the hidden single, leading to corrupted notes.
- Assuming a cell with three candidates cannot be placed yet, without checking if one of those candidates is unique to the house.
Frequently asked questions
- Why is it called 'Hidden' Single?
- Because the placed number is 'hidden' among other candidates in the same cell. If a cell has candidates {2, 5, 8}, but 5 cannot go anywhere else in that row, 5 is a Hidden Single.
- What is cross-hatching in Sudoku?
- Cross-hatching is the visual technique of tracking a single digit across adjacent boxes and drawing imaginary lines through rows and columns to pinpoint where that digit must land in a target box.
- Should I look for Naked Singles or Hidden Singles first?
- Most solvers find Hidden Singles first using visual cross-hatching because it requires zero pencil marks. When cross-hatching stalls, pencil marks are added to reveal Naked Singles and Subsets.
Related techniques
Put Hidden Single into practice
Try a puzzle from the Gentle tier (Peak Score 11–20).