Beginner Technique

Obvious Singles

The most fundamental Sudoku solving technique. Find cells where only one number can possibly fit.

What Are Obvious Singles?

An Obvious Single (also called a "Naked Single") occurs when a cell can only contain one possible number. This happens when the other eight numbers already appear in the same row, column, or 3×3 box.

Key Point

Obvious Singles are the building blocks of Sudoku solving. Every puzzle contains them, and they're often your starting point for more advanced techniques.

How to Identify Obvious Singles

The Process

  1. 1 Choose an empty cell
  2. 2 Check which numbers 1-9 already exist in its row
  3. 3 Check which numbers already exist in its column
  4. 4 Check which numbers already exist in its 3×3 box
  5. 5 If only one number is missing from all three constraints, you found an Obvious Single!

Quick Check Method

For each empty cell, ask yourself: "What numbers can't go here?" If 8 out of 9 numbers are eliminated, the remaining number is your answer.

💡 Pro Tip: Start with cells in nearly complete rows, columns, or boxes for easier identification.

Example Walkthrough

Step 1: Identify the Target Cell

Look at the highlighted cell in the puzzle below. We need to determine what number can go here.

Let's examine the constraints:

  • • Row contains: 5, 3, 7
  • • Column contains: 6, 8
  • • Box contains: 5, 3, 6, 9, 8
5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
3
1
6
8
4
1
9
8
7
9
7
9
6
2
8
4
1

Target cell highlighted in yellow

Step 2: Eliminate Impossible Numbers

Let's list all numbers that are already used in the row, column, and box that contains our target cell:

Row 1:

Contains: 3, 5, 7

Column 3:

Contains: 6, 8

Top-left box:

Contains: 3, 5, 6, 8, 9

Numbers eliminated:

3, 5, 6, 7, 8, 9

Numbers still possible:

1, 2, 4

Step 3: Check for Additional Constraints

Wait! Let's double-check our work. Looking more carefully at the constraints:

  • • Row 1 already has: 3, 5, 7
  • • Column 3 already has: 6, 8
  • • Top-left box already has: 3, 5, 6, 8, 9

The union of all constraints eliminates: 3, 5, 6, 7, 8, 9

Only possible numbers: 1, 2, 4

This is NOT an Obvious Single!

Since multiple numbers (1, 2, 4) are still possible, this cell is not an Obvious Single. We need more information or different techniques to solve it.

Better Example: True Obvious Single

Let's look at a different cell that IS an Obvious Single:

In this row, we can see that numbers 1, 2, 3, 4, 5, 6, 7, 9 are already present.

The only missing number is 8!

This empty cell must contain 8.

1
2
3
4
5
6
7
?
9

Row missing only the number 8

When to Use Obvious Singles

Best Scenarios

  • ✓ At the beginning of any puzzle
  • ✓ When rows, columns, or boxes are nearly complete
  • ✓ After applying other techniques that fill in numbers
  • ✓ When you're stuck with more advanced techniques

Strategy Tips

  • 💡 Scan systematically: check every empty cell
  • 💡 Focus on regions with fewer empty cells
  • 💡 After finding one, check for new Obvious Singles
  • 💡 Use this technique before trying complex methods

Practice Makes Perfect

How to Master This Technique

  1. 1 Start with easy puzzles to build pattern recognition
  2. 2 Practice scanning rows, columns, and boxes quickly
  3. 3 Time yourself to improve scanning speed
  4. 4 Always double-check your work before placing numbers

Common Mistakes to Avoid

  • ✗ Forgetting to check all three constraints (row, column, box)
  • ✗ Moving too quickly and missing obvious singles
  • ✗ Confusing obvious singles with hidden singles
  • ✗ Not rechecking after placing new numbers
Previous: Basic Rules Next: Hidden Singles

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