Two digits that can only go in two cells within a unit. Strip away everything else.
Pick a unit and ask which cells each digit can still go in. When two digits are restricted to the same two cells, those cells belong to that pair alone, so strip out every other candidate from them. The result is a Naked Pair that often unlocks further eliminations.
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}).
Look at digits rather than cells. Take one unit and, for each unplaced digit, list the cells that can still hold it. If two digits map to exactly the same two cells and appear nowhere else in that unit, you have a Hidden Pair. Pencil marks make this scan far easier.
Yes, and that is exactly what makes the pair hidden. One cell might read {1, 3, 5, 8} and its partner {3, 5, 7, 9}. If 3 and 5 appear in no other cell of the unit, they are a Hidden Pair, and the 1, 8, 7 and 9 all come out.
A Naked Pair is two cells that hold the same two candidates and nothing else, so you remove those two digits from the rest of the unit. A Hidden Pair is two digits confined to two cells that may hold extra candidates, so you remove the extras from the pair’s cells instead.
Then it is not a Hidden Pair and the elimination is invalid. Both digits must be confined to the same two cells within the unit. If digit 3 can also go in a third cell, 3 might land there instead, and stripping candidates from your two cells would remove a digit the solution needs.
No. A Hidden Pair never places a digit on its own; it only clears the extra candidates out of the two cells. What you gain is simplification: the two cells become a clean Naked Pair, which frequently exposes Naked Singles or Locked Candidates elsewhere. Re-scan the affected row, column and box afterwards.
Rows and columns, where the cells lie in a straight line and the digit-by-digit scan is easy to follow. Hidden Pairs also occur in boxes, where the logic is identical but the three-by-three layout makes them spatially harder to see. They turn up in Medium and Hard puzzles, often alongside Locked Candidates.
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