Beginner Technique

Last Possible Number

Systematically eliminate candidates until only one number remains possible for a cell.

What Is Last Possible Number?

The Last Possible Number technique, also known as "Naked Singles," involves systematically eliminating possible numbers for a cell until only one candidate remains. This is a process of elimination that focuses on individual cells rather than groups.

Key Point

For any empty cell, start with all possible numbers (1-9) and eliminate those that already appear in the same row, column, or 3x3 box. When only one number remains possible, that's your answer.

How It Works

The Process

  1. 1 Choose an empty cell to analyze
  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 3x3 box
  5. 5 If only one number is not eliminated, place it in the cell

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.

This technique is also called "Naked Singles" or "Obvious Singles" because the solution becomes obvious once you eliminate impossible values.

Example Walkthrough

Step-by-Step Elimination

Let's find what number goes in a cell by systematically eliminating candidates:

5
3
?
8
1
9
2
6
4

Row contains: 1, 2, 3, 4, 5, 6, 8, 9. Only 7 is missing!

Check Row

Contains: 1, 2, 3, 4, 5, 6, 8, 9

Eliminates: 1, 2, 3, 4, 5, 6, 8, 9

Check Column

If 7 doesn't appear in column

7 remains possible

Check Box

If 7 doesn't appear in box

7 remains possible

Result

After checking all constraints, only 7 can go in the highlighted cell. This is a Last Possible Number!

When to Use This Technique

Best Scenarios

  • ✓ Starting any puzzle - always check first
  • ✓ After placing any number - new eliminations may occur
  • ✓ Cells in nearly complete rows, columns, or boxes
  • ✓ When stuck - scan all empty cells systematically

Common Mistakes

  • ✗ Forgetting to check all three constraints
  • ✗ Moving too fast and missing obvious singles
  • ✗ Not rechecking after placing new numbers
  • ✗ Skipping cells that "look" complicated

Practice Tips

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 Use pencil marks to track possible candidates
  4. 4 Always verify your answer before placing a number

Related Techniques

Previous: Basic Rules Next: Obvious Singles

Ready to Practice?

Try finding Last Possible Numbers in your next puzzle - scan each empty cell systematically!

Practice Now