Hidden pairs in Sudoku: two digits, two possible cells
Follow one digit across a complete unit, then repeat with a second digit. If both searches end in the same two cells, those cells are reserved for the two tracked digits.
Definition
A hidden pair is two digits that can appear only in the same two cells of a unit; every other candidate can be removed from those two cells.
When to use it and what to verify
When to look for it
Follow two missing digits across an entire unit when their candidate positions are sparse but the target cells contain extra notes.
Required condition
Both digits must be legal in the same two cells and nowhere else in the row, column, or box.
Exact elimination boundary
Keep the hidden pair and remove only the other candidates from those two cells. Do not remove the pair digits from their reserved cells.
Candidates 2 and 9 can appear only at R1C2 and R1C6.
- 1. Map candidate 2In row 1, candidate 2 appears only at R1C2 and R1C6.
- 2. Map candidate 9Candidate 9 ends in the same two cells.
- 3. Reveal the pairKeep 2 and 9. Remove 5 from R1C2 and 6 from R1C6.
Row 1 must place both 2 and 9 in R1C2 and R1C6, so candidate 5 leaves R1C2 and candidate 6 leaves R1C6.
A reference verified by Doku.su
Doku.su checks every placement or elimination in this example with its deterministic Sudoku engine. Illustrative candidates are labelled when they do not represent a complete puzzle state.
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