Two digits that can only go in two cells within a unit. Strip away everything else.
A Hidden Pair occurs when two digits appear as candidates in exactly the same two cells within a unit, and nowhere else in that unit. Since both digits must go in those two cells, all other candidates in those cells can be removed.
The pair is "hidden" because the two cells typically contain additional candidates that obscure the pattern. A cell might show {1, 3, 5, 8} while its partner shows {3, 5, 7, 9}. The digits 3 and 5 only appear in these two cells within the unit. That's your Hidden Pair. Remove everything except 3 and 5 from both cells.
Hidden Pairs are harder to spot than Naked Pairs because you're looking for what's exclusive to two cells rather than what's common between two minimal cells. They appear in Medium and Hard puzzles, often alongside Locked Candidates.
Instead of looking at cells (like you do for Naked Pairs), look at digits. Pick a unit, say box 5, and ask: "For each digit, which cells can contain it?" If two digits share exactly the same two cells and appear nowhere else in that unit, you've found a Hidden Pair.
Suppose in box 5, digit 2 can only go in R4C5 or R6C4, and digit 8 can also only go in R4C5 or R6C4. No other cell in box 5 has 2 or 8 as a candidate. Since both 2 and 8 must go in these two cells, remove all other candidates from R4C5 and R6C4. They become {2, 8} each, which is now a Naked Pair.
This is the key insight: a Hidden Pair becomes a Naked Pair after you clean up the extra candidates. The elimination doesn't directly place digits, but it simplifies the cells dramatically, often triggering further techniques.
You can find Hidden Pairs in rows, columns, or boxes. Check each digit's candidate positions within a unit. If two digits always share the same two positions, that's your pair.
Example 1: Hidden Pair in a Row
Examine row 6 and map where each unplaced digit can go. Digit 6 can only appear in R6C1 and R6C7. Digit 8 can also only appear in R6C1 and R6C7. No other cell in row 6 has 6 or 8 as a candidate.
Since digits 6 and 8 must occupy R6C1 and R6C7, all other candidates in those cells can be removed. R6C1 has {4, 6, 8}, remove 4, leaving {6, 8}. R6C7 has {6, 7, 8, 9}, remove 7 and 9, leaving {6, 8}.
Both cells now show {6, 8}, a clean Naked Pair. The Hidden Pair has simplified two busy cells, potentially unblocking further techniques in the intersecting columns and boxes.
Example 2: Hidden Pair in a Box
Now look at box 7 (bottom-left). Map digit positions: digit 2 can only go in R7C3 and R8C1. Digit 8 can also only go in R7C3 and R8C1. No other cell in box 7 has 2 or 8.
Since 2 and 8 are confined to these two cells, remove all other candidates. R7C3 has {2, 4, 6, 7, 8}, remove 4, 6, and 7, leaving {2, 8}. R8C1 has {2, 7, 8}, remove 7, leaving {2, 8}.
The pair cells are now simplified to {2, 8}. Notice how the pair was "hidden" among many other candidates, you wouldn't have spotted it by looking at the cells alone.
Example 3: Hidden Pair in a Column
In column 4, digits 1 and 4 can only go in two cells: R5C4 and R8C4. No other cell in column 4 has either 1 or 4 as a candidate. Since both digits must go in these two cells, remove all other candidates: remove 9 from R5C4 (was {1, 4, 9}) and remove 7 from R8C4 (was {1, 4, 7}).
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