Hidden Single
A digit that can only go in one cell of a house, even if other candidates share the cell.
Also known as: Unique Candidate, Penciled Single, Pinned Digit
Quick Summary
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.
Understanding the Hidden Single Technique
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.
Visual Examples
Green cells highlight the Hidden Single pattern, amber cells denote where candidates are affected, and struck-through pencil marks show the eliminations. Use the buttons or arrow keys to browse examples.
How to Spot & Solve a Hidden Single (Step-by-Step Guide)
The complete visual scanning and logical deduction process to execute this technique:
- 1Choose a row, column, or 3×3 box and pick a digit you are tracking.
- 2Scan every empty cell in that unit to see if intersecting lines already block that digit from entering those cells.
- 3Cross out all blocked cells from consideration.
- 4If 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 Traps & Mistakes to Avoid
- 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 Solving Techniques
Expand your repertoire by learning prerequisites and advanced counterparts:
Ready to Practice Hidden Single?
Try a puzzle in the Gentle tier (Peak Score 11–20), or browse all 25 techniques.