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Simply click on any empty cell and select a number from 1-9. Use logic to determine which numbers can go in each cell based on Sudoku rules - each row, column, and 3x3 box must contain all numbers 1-9 exactly once.

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What Is a Hidden Single in Sudoku? A Complete Beginner-Friendly Guide

Introduction

Hidden Singles are one of the most important early-game techniques in Sudoku.

They appear in nearly all puzzles—from Easy all the way to Expert—and recognizing them quickly can dramatically speed up your solving.

In this guide, we'll explain what a Hidden Single is, how to identify it, and how you can use it to solve puzzles faster on SudokuGames.org. Understanding Hidden Singles is essential for progressing from beginner to intermediate level.

Key Points

Master these fundamentals to understand Hidden Singles:

Hidden Singles occur when a number can go in only one place within a row/column/box: Unique placement within a unit
They often appear early in the puzzle after pencil marks are added: Notes reveal hidden placements
Essential for beginners who want to progress to Medium & Hard levels: Foundation technique for advancement
One of the fastest and most reliable techniques: Immediate, guaranteed placements
Appears in virtually all Sudoku puzzles: Universal technique across difficulty levels

What Is a Hidden Single?

A Hidden Single is a situation where a number has only one valid location in a:

row
column
or 3×3 box

Even if the cell has multiple pencil marks, logic reveals: Only one cell can hold a certain number.

It's "hidden" because:

all cells may have multiple candidates — not obvious at first glance
the correct one isn't obvious until you check the entire unit — requires scanning

What Makes a Hidden Single Different?

Hidden Singles differ from Naked Singles:

Naked Single: Only ONE candidate left in a cell. The cell clearly shows the answer.

Hidden Single: A number appears only once across a unit, even if the cell has multiple candidates. The placement is "hidden" within candidate lists.

Both are essential, but Hidden Singles require more awareness and systematic scanning.

How It Works (Step-by-Step)

Identifying Hidden Singles requires systematic scanning:

Step 1: Add Pencil Marks

List all candidates for empty cells. Notes Mode on SudokuGames.org makes this automatic. Without notes, Hidden Singles are nearly impossible to detect.

Step 2: Scan Each Unit (Row, Column, Box)

Look for a candidate number that appears only once. Check each number from 1-9 systematically across the entire unit.

Step 3: Confirm Placement

If a "9" appears in only one cell of row 7 → place it. The uniqueness guarantees this is the correct placement.

Step 4: Update Notes

Remove the number from the row/column/box. This often reveals more Hidden Singles through cascading eliminations.

Once identified, Hidden Singles often cause cascades that reveal more Singles. One Hidden Single can unlock multiple subsequent placements.

Why Hidden Singles Matter

Hidden Singles are foundational. They help you:

make steady progress without guessing — guaranteed logical placements
simplify complex puzzles — reduce candidate clutter
unlock Medium → Hard difficulty levels — essential technique for progression
eliminate branches of incorrect guesses — prevent errors through logic
keep solving momentum — continuous progress through puzzle

On SudokuGames.org, Hidden Singles become much easier to detect with the Notes feature enabled.

How to Identify a Hidden Single

Use this systematic approach:

Systematic Scanning Method

Scan by number, not by cell:

Choose a number (e.g., "5")
Check every cell in a row/column/box for "5" as a candidate
If "5" appears in only one cell → Hidden Single found
Place "5" immediately
Remove "5" from related units
Repeat for all numbers 1-9

Example Detection

Imagine a 3×3 box with these candidates:

{1,3} {3} {1,3,5}

{5,6} {1,6} {5}

{1,2} {1,3} {2,3}

Scanning for "6": Only one cell contains the candidate 6.

Therefore, 6 is a Hidden Single and must go in that cell.

Examples

Here are practical examples demonstrating Hidden Singles:

Example 1: Row Hidden Single

In row 4, number 2 appears only once in the candidate lists → place 2 immediately. Even though the cell may have other candidates like {2,7,9}, the "2" must be placed because it can't go anywhere else.

Result: Immediate, guaranteed placement.

Example 2: Column Hidden Single

Column 6 has only one cell containing candidate 7 → place 7. The column scan reveals the unique placement clearly.

Result: Systematic scanning reveals hidden placements.

Example 3: Box Hidden Single

A 3×3 box has only one cell containing 9 in all candidate lists → place 9. Box-level Hidden Singles are common and powerful.

Result: Notes reveal box-level hidden placements.

How Hidden Singles Help You Solve Faster

Hidden Singles typically:

unlock rows/columns that were previously stuck — break through dead ends
trigger new deductions — cascading eliminations
reduce future branching — fewer possibilities
reveal advanced techniques — set up Pairs/Triples/X-Wing patterns

They are especially common in the early–mid stage of Medium and Hard puzzles.

Summary

Hidden Singles are one of the most powerful beginner techniques in Sudoku.

By learning to recognize them, you will solve puzzles faster, avoid guessing, and prepare for more advanced logic. Turn on Notes mode on SudokuGames.org to spot Hidden Singles with ease.

Mastering Hidden Singles creates a solid foundation for learning intermediate and advanced techniques. This fundamental skill supports progression through all difficulty levels.

❓ FAQ

Q1: Are Hidden Singles common?

Yes, they appear frequently—even in advanced puzzles. Hidden Singles are universal across all difficulty levels, making them essential knowledge.

Q2: Do I need pencil marks to spot them?

Yes, pencil marks make Hidden Singles much easier to identify. Without notes, Hidden Singles are nearly impossible to detect reliably.

Q3: Do experts still rely on Hidden Singles?

Absolutely. They speed up solving even for high-level players. Hidden Singles are efficient, so experts use them throughout solving.

Q4: Are Hidden Singles enough to solve Hard puzzles?

They're essential but not sufficient. Hard puzzles require Pairs/Triples and advanced logic. Hidden Singles provide foundation but need support from other techniques.

Q5: Does SudokuGames.org support pencil marks?

Yes. There is a note-taking mode specifically designed to help spot Hidden Singles. Notes Mode makes detection much easier.

Q6: How do Hidden Singles differ from Naked Singles?

Naked Singles have only one candidate in the cell itself. Hidden Singles have the number appearing only once in the unit, even if the cell has multiple candidates.

Q7: Can I solve Easy puzzles without knowing Hidden Singles?

Yes, but you'll progress much faster with Hidden Singles. They're essential for Medium+ difficulty and dramatically improve solving efficiency.

Q8: Why are they called "hidden"?

They're "hidden" because the cell may have multiple candidates, hiding the fact that one number must be placed. Systematic scanning reveals the hidden placement.

Ready to master Hidden Singles? Start practicing with Notes Mode enabled today!

Play Sudoku Now | Learn More Solving Techniques

Visit SudokuGames.org to practice Hidden Singles detection, explore our technique guides for more strategies, and track your progress as you improve!