Hidden Pair
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Hidden Pair is a size-2 subset.
How to spot it
- Examine each house (row, column, and box) in turn. Choose two distinct digits that have not yet been placed in the house,
aandb, and examine their candidate positions. - If all possible positions for those two digits in the house together occupy exactly two unfilled cells, and at least one of those cells contains other candidates, eliminate all candidates other than
aandbfrom those two cells.
How it works
- Each of
aandbmust appear exactly once in the house. Because they can appear only in those two cells, they must occupy all two cells, one digit in each. - Placing any other digit in one of those cells would leave too few cells for
aandb. Therefore, all other candidates can be eliminated from those two cells.
Examples
A hidden pair can appear in a row. For example, in row 4 below, digits 1 and 5 can appear only in R4C2 and R4C4, so all other candidates can be eliminated from those two cells.
A hidden pair can also appear in a column. For example, in column 6 below, digits 4 and 7 can appear only in R4C6 and R6C6, so all other candidates can be eliminated from those two cells.
A hidden pair can also appear in a box. For example, in box 1 below, digits 1 and 4 can appear only in R2C3 and R3C1, so all other candidates can be eliminated from those two cells.