"Obvious Pairs" Technique
What is an "Obvious Pairs"?
An "Obvious Pair" (also commonly known as a "Naked Pairs") is a powerful, advanced technique based on using Notes (candidates).
This pattern occurs when two cells within the same house (a 3x3 block, row, or column) contain only the exact same two candidate notes.
The Logic Behind It
The logic is: if Cell A and Cell B can only be a '7' or a '9', then the numbers '7' and '9' are locked into those two cells.
Therefore, '7' and '9' cannot possibly exist in any other cell within that same house (block, row, or column). Our job is to use this "lock" to eliminate candidates from other cells.
How to Use the "Obvious Pairs" Technique
Let's look at a perfect example in the bottom-middle 3x3 block.
In this block, we can see the following filled numbers and notes:
- Filled Numbers: 1 (R7C5), 2 (R7C6), 3 (R8C5), 5 (R9C4), 8 (R9C5).
- Notes (Candidates):
R7C4 = {7, 9}
R8C4 = {4, 7, 9}
R8C6 = {7, 9}
R9C6 = {6, 7, 9}
Step 1: Find the Pair
Look closely at the notes. We can identify:
- Cell R7C4 contains only {7, 9}.
- Cell R8C6 also contains only {7, 9}.
These two cells, being in the same block, form a perfect "Obvious Pair".
Step 2: Eliminate Candidates
Based on our logic, since 7 and 9 are locked into R7C4 and R8C6, they cannot appear in any other cell in this block.
- We must now remove the '7' and '9' notes from all other cells in this block.
- For R8C4: Its notes are {4, 7, 9}. After removing 7 and 9, only {4} remains.
- For R9C6: Its notes are {6, 7, 9}. After removing 7 and 9, only {6} remains.
Step 3: Find the New Solutions
By performing this elimination, we have instantly revealed two new "Obvious Singles"!
- Cell R8C4 must now be 4.
- Cell R9C6 must now be 6.
Master This Advanced Technique
This is the power of the "Obvious Pairs" technique. It helps players find a breakthrough when the puzzle seems stuck. Once mastered, it will make solving Hard and Expert level Sudoku puzzles much faster!