Introduction

Obvious Pairs (also called Naked Pairs) is a powerful intermediate Sudoku technique that enables significant eliminations in medium to hard puzzles. Unlike Hidden Pairs, which are subtle, Obvious Pairs are visually apparent—two cells contain only two candidates total. This makes them easier to spot and apply. This guide provides everything you need to master this essential technique through clear explanations, step-by-step instructions, and detailed examples.

What Is the Obvious Pairs Technique

Obvious Pairs is an elimination technique that works when two cells in the same unit (row, column, or box) contain only two candidates total, in various combinations. For example, two cells might both contain only candidates 3 and 7. Or one cell might have 3 and 7, while the other has just 7 (but then it's not a pair—both must have the same two candidates).

Actually, for Obvious Pairs, both cells must contain exactly the same two candidates. For example, cell 1 has candidates 3 and 7, and cell 2 also has candidates 3 and 7. Together, these two cells contain only candidates 3 and 7.

The key insight is that these two cells must contain those two candidates (in some order). This means those two candidates cannot appear in any other cells in that unit, allowing elimination. The technique is called "obvious" because the pattern is visually apparent—you can see at a glance that two cells contain only two candidates.

Key Points

1. Two Cells, Two Candidates: Obvious Pairs require two cells containing exactly the same two candidates (no other candidates present).

2. Visually Obvious: Unlike Hidden Pairs, the pattern is visually apparent—you can see the two cells contain only two candidates.

3. Forced Placement: The two candidates must occupy those two cells, enabling elimination from other cells in the unit.

4. Elimination Power: You can eliminate the two candidates from all other cells in that unit.

5. Candidate Notation Required: While obvious once found, Obvious Pairs require candidate notation to identify.

How to Identify and Apply Obvious Pairs

Step 1: Set Up Complete Candidate Notation

Obvious Pairs identification requires complete candidate notation. Ensure every empty cell has all possible candidates marked. Incomplete notation will cause you to miss Obvious Pair opportunities.

Step 2: Scan Units for Pair Patterns

Scan each unit (row, column, box) looking for two cells that both contain exactly the same two candidates and no other candidates. Look for cells where you can see at a glance that they form a pair. The pattern should be visually obvious.

Step 3: Verify the Pattern

Confirm that both cells contain exactly the same two candidates and no other candidates. Check that no additional candidates are present in either cell. If other candidates are present, it's not a valid Obvious Pair. Verification ensures accurate identification.

Step 4: Apply Elimination

Once verified, eliminate the two candidates from all other cells in that unit. These eliminations are logically forced because the two candidates must occupy the two pair cells, leaving no room for them elsewhere in the unit.

Step 5: Update and Continue

After making eliminations, update your candidate notation throughout the puzzle. The new eliminations may reveal additional placements or create opportunities for other techniques. Continue scanning for more Obvious Pairs or other solving opportunities.

Examples

Example 1: Obvious Pair in a Row

In row 5, cells R5C2 and R5C8 both contain only candidates 3 and 7. No other candidates are present in these cells. This forms an Obvious Pair. These two cells must contain 3 and 7 (in some order), so we can eliminate 3 and 7 from all other cells in row 5. If R5C4 had candidates 2, 3, and 7, we would eliminate 3 and 7, leaving only 2. This might reveal that R5C4 must be 2.

Example 2: Obvious Pair in a Box

In the top-left box, cells R1C1 and R2C3 both contain only candidates 4 and 9. This forms an Obvious Pair. We eliminate 4 and 9 from all other cells in that box. This elimination may reveal new placements or enable other techniques.

Example 3: Obvious Pair Leading to Progress

After identifying an Obvious Pair and eliminating candidates from other cells in the unit, one of those other cells might now have only one remaining candidate, allowing immediate placement. This cascading effect is common with Obvious Pairs, where one elimination reveals multiple solving opportunities.

Summary

Obvious Pairs is an essential intermediate technique that enables significant eliminations in medium to hard puzzles. The key to success is scanning candidate notation to identify when two cells contain exactly the same two candidates. Once found, Obvious Pairs provide powerful eliminations that progress puzzles significantly. Practice is essential for developing the pattern recognition skills needed to spot these patterns quickly.

The technique works best when you have complete candidate notation and scan methodically through each unit. Don't rush—careful scanning prevents missing Obvious Pairs and ensures accurate identification. As you practice, you'll develop an eye for spotting these patterns more quickly. Obvious Pairs often work in combination with other techniques, creating cascading eliminations that significantly progress puzzle solving. Master this technique to improve your efficiency with intermediate puzzles and prepare for more advanced methods.

FAQ (Frequently Asked Questions)

Q1: What's the difference between Obvious Pairs and Hidden Pairs?

Obvious Pairs (Naked Pairs) are visually apparent: two cells contain only two candidates (no other candidates present). Hidden Pairs are less obvious: two candidates appear only in two cells, but those cells also contain other candidates. Obvious Pairs are easier to spot visually, while Hidden Pairs require scanning candidate frequencies. Both enable the same eliminations.

Q2: Do both cells need to have exactly the same two candidates?

Yes, for Obvious Pairs, both cells must contain exactly the same two candidates and no other candidates. For example, both cells must have candidates 3 and 7, and nothing else. If one cell has 3, 7, and 9, it's not an Obvious Pair. The pattern requires precise matching: two cells, two candidates, identical in both cells.

Q3: How do I know if I've found a valid Obvious Pair?

A valid Obvious Pair requires: (1) exactly two cells in the same unit, (2) both cells contain exactly the same two candidates, and (3) no other candidates are present in either cell. If you find two cells that both contain only the same two candidates, you've found an Obvious Pair.

Q4: Can Obvious Pairs work with more than two candidates?

No, Obvious Pairs specifically require exactly two cells containing exactly two candidates. If the cells contain three or more unique candidates, it's not a valid Obvious Pair. The pattern requires precise matching: two cells, two candidates.

Q5: Are Obvious Pairs necessary for all medium puzzles?

Not all medium puzzles require Obvious Pairs, but many do. Obvious Pairs become necessary when basic techniques have been exhausted but the puzzle isn't hard enough to require advanced methods. If you're stuck on a medium puzzle and basic scanning reveals no opportunities, look for Obvious Pairs. They're often the next logical step.

Q6: How do I avoid missing Obvious Pairs?

Systematic scanning is key. Scan each unit (row, column, box) methodically, looking for pairs of cells that both contain only two candidates. Look for cells with few candidates first, as these are more likely to be part of pairs. Don't rush—careful scanning ensures you don't miss patterns.

CTA (Call to Action)

Ready to Master Obvious Pairs?

Practice identifying and applying Obvious Pairs with our free online Sudoku puzzles. Start with medium difficulty puzzles where this technique becomes essential.

Get Started Today:

• Play Sudoku Now - Practice Obvious Pairs on medium difficulty puzzles
• Download Printable Puzzles - Solve offline and master the technique
• Learn More Techniques - Explore additional strategies

For related techniques, check out our guides on Hidden Pairs and Intermediate Techniques.

---