Sudoku Techniques: Pairs (Naked Pair and Hidden Pair)
Use two cells that share the same two candidates to eliminate candidates from other cells.
Naked pair
Suppose two cells in the same row (or column, or box) have exactly the same two candidates, say 3 and 7. These two cells will share the 3 and the 7 between them, so you can remove 3 and 7 from the candidates of every other cell in that row.
Hidden pair
Suppose that within a row (or column, or box), two numbers A and B can go in only the same two cells. Those two cells must hold A and B, so you can remove all their other candidates.
How to spot them
Write your notes and look for the same combination in two cells. When you find one, remove those two numbers from the other cells in the same group. This often leaves a cell with only one possible number.
More articles: Sudoku Rules and How to Get Started / The Two Basic Sudoku Techniques: Singles (One Number per Cell, One Cell per Number) / How to Use Notes (Candidates) in Sudoku / Sudoku Techniques: Boxes and Lines (Pointing Pairs) / Sudoku Techniques: X-Wing / Sudoku Techniques: Trial and Error (for Demon, Hell and Abyss) / How Kids Can Play Sudoku (6×6)
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