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Hidden Single technique in Sudoku
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Hidden Single

Find Where a Number Must Go

By Minimal Sudoku TeamLast updated:

The Hidden Single is one of the most powerful beginner techniques in Sudoku. Instead of asking "what can go in this cell?", it asks "where can this number go in this row/column/box?"

Quick Summary
  • What: A digit that can only appear in one cell within a row, column, or box
  • How: Check where a specific digit can go — if only one cell works, that's the answer
  • Difficulty: Beginner — essential technique for all puzzles
  • Also called: Pinned Digit, Unique Position

The Concept

Every row, column, and 3×3 box must contain each digit from 1 to 9 exactly once. The Hidden Single technique uses this rule in reverse: if a digit can only go in one cell within a unit, it must go there.

The digit is "hidden" because the cell might have other candidates too — it doesn't look special at first glance. But within its row, column, or box, it's the only cell where that digit can appear.

The Golden Rule

🔎 Hidden Single Rule
If a digit can only appear in one cell within a row, column, or 3×3 box, that cell must contain that digit — regardless of what other candidates the cell may have.

How to Find Hidden Singles

1

Pick a unit to analyze

Choose a row, column, or 3×3 box. Units with more filled cells are often easier to analyze.
2

Pick a digit (1-9)

Choose a digit that hasn't been placed in this unit yet.
3

Find all possible cells

Check each empty cell in the unit: could this digit go here? Eliminate cells where the digit appears in the same row, column, or box.
4

Count the possibilities

If only one cell can contain the digit, you've found a Hidden Single!
5

Place the digit

The digit must go in that cell. Place it and look for chain reactions.
Systematic Scanning
Work through digits 1-9 systematically for each unit. This ensures you don't miss any Hidden Singles. Many solvers call this "scanning" — checking where each digit can go.

Practical Example

Finding a Hidden Single in a Box

Hidden Single example in a 3×3 box

Let's look at the center box and try to place the digit 7.

First, we identify empty cells in the box. Then we check which ones can contain 7:

  • R4C4: 7 appears in row 4 → eliminated
  • R4C6: 7 appears in row 4 → eliminated
  • R5C5: Already filled
  • R6C4: 7 appears in column 4 → eliminated
  • R6C5: No conflicts → possible!
  • R6C6: 7 appears in column 6 → eliminated
Hidden Single Found
Only R6C5 can contain 7 in this box. Place it there!

Three Types of Hidden Singles

Hidden Singles can occur in any of the three unit types:

Row Hidden Single

A digit can only go in one cell within a row.

"Where can 5 go in row 3?"

Column Hidden Single

A digit can only go in one cell within a column.

"Where can 5 go in column 7?"

Box Hidden Single

A digit can only go in one cell within a 3×3 box.

"Where can 5 go in the center box?"

Box Singles are Most Common
In typical puzzles, you'll find more Hidden Singles in boxes than in rows or columns. Boxes are more constrained because they overlap with multiple rows and columns.

Hidden Single vs Naked Single

These two techniques complement each other perfectly:

Naked Single

Focus: The cell

"What can go in this cell?" → Only one option left.

Hidden Single

Focus: The digit

"Where can this digit go?" → Only one cell available.

A good solver uses both techniques together. When one doesn't find anything, switch to the other!

Naked SingleBeginner

Naked Single

Learn the complementary technique: finding cells where only one digit is possible.

Detection Tips

Start with Near-Complete Units
A row, column, or box with 7-8 filled cells only has 1-2 empty cells. It's trivial to check which digits can go where.
Cross-Hatch Scanning
For each digit, visualize "lines" extending from every instance of that digit. Empty cells that intersect with all lines except one are Hidden Singles.
Use Snyder Notation
Snyder Notation marks candidates when a digit can only go in 2 cells within a box. When one is eliminated, the remaining mark becomes a Hidden Single!
Don't Overlook Rows and Columns
Many solvers focus only on boxes. Remember to check rows and columns too — Hidden Singles appear in all three unit types.

Related Techniques

Beginner Techniques

1️⃣Naked Single📍Last Remaining Cell👀Scanning Technique

Level Up

👀Hidden Pairs✏️Snyder Notation👉Pointing Pairs