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Hidden pairs and triples: digits reserve cells
A naked pair starts with cells that have few candidates. A hidden pair starts with digits that have few possible positions, even among crowded notes.
Check the entire unit
Within a row, column or box, two missing digits restricted to two cells reserve those cells. Remove other candidates from them. A hidden triple uses three digits restricted to three cells; each cell need not contain all three digits.
HoDoKu — Hidden Subsets · SudokuWiki — Hidden Candidates
Crossed-out digits are eliminations. Check the full conditions in the worked example below.
A pair with different removals
Let A1={1,4,7} and A5={1,4,8}, with no other positions for 1 or 4 in row A. These cells must share 1 and 4. Remove 7 from A1 and 8 from A5, keeping the hidden pair itself.
Try before revealing
Does the same hidden pair work if A9 also permits 1?
Show explanation
No. One of its digits now has a position outside the two source cells. This argument no longer establishes the eliminations.
Extend to three digits
Suppose C1={2,5,9}, C4={2,6,8}, C8={5,6,7}, and row C has no other positions for 2, 5 or 6. Remove 9, 8 and 7 from those cells respectively. Their individual answers remain unresolved.
Use it in your game
The Techniques tool detects both sizes from actual entries. A source can also be an elimination target: follow the crossed-out candidates and the per-cell removal list. Your handwritten notes remain unchanged.
References and further reading
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