Naked Pair
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Naked Pair is a size-2 subset.
How to spot it
- Examine each house (row, column, and box) in turn. Consider groups of two unfilled cells (with no given or entered digit) and examine their candidates.
- If the candidates across those two cells consist of exactly two distinct digits,
aandb, eliminate those candidates from the other cells in the house.
How it works
- Those two cells can contain only
aandb. Because digits cannot repeat within a house, the cells must contain those two digits, one in each cell, although their exact arrangement is not yet known. - Therefore, no other cell in the house can contain any of those two digits, so those candidates can be eliminated from the other cells.
Examples
A naked pair can appear in a row. For example, in row 6 below, cells R6C6 and R6C7 contain only candidates 2 and 5, so those candidates can be eliminated from the other cells in the row.
A naked pair can also appear in a column. For example, in column 6 below, cells R5C6 and R7C6 contain only candidates 3 and 8, so those candidates can be eliminated from the other cells in the column.
A naked pair can also appear in a box. For example, in box 7 below, cells R7C3 and R9C1 contain only candidates 1 and 5, so those candidates can be eliminated from the other cells in the box.