The Hidden Pair strategy is a powerful technique used when two specific numbers appear as candidates in exactly two cells within the same row, column, or 3x3 box.
Unlike a Naked Pair, where the cells contain only the two numbers, a Hidden Pair is "hidden" because the cells contain other candidates as well. However, since the two numbers don't appear anywhere else in the unit, they must go in these two cells.
Interactive Example
Click "Apply Logic" to see the strategy in action.
How it Works
- Identify a Unit: Look at a single row, column, or box.
- Count Candidates: Notice that two specific numbers (e.g., 1 and 7) appear in only two cells in that unit.
- The Logic: Since 1 and 7 must be placed in that unit, and they only fit in those two specific spots, those two spots are reserved for 1 and 7.
- The Result: You can eliminate all other candidates from those two cells.
Example Explanation
In the example above, look at Row 7.
* The candidates 1 and 7 are only valid in two cells (R7C4, R7C6).
* Check the rest of the row: 1 and 7 are not present as candidates anywhere else.
* Therefore, 1 and 7 form a Hidden Pair in R7C4 and R7C6.
* We can remove the extra candidates (like 2) from these two cells, leaving only 1 and 7.
Why is it called "Hidden"?
It's called "Hidden" because the pair isn't obvious at first glance. The cells are cluttered with other candidates. You interpret the clue by looking at the entire unit to spot that the numbers have nowhere else to go.
Once you find a Hidden Pair, you essentially convert it into a Naked Pair by stripping away the extra candidates.