Naked pairs in Sudoku: reserve two digits in two cells
Compare complete candidate lists in one row, column, or 3×3 box. Two cells reserve two digits only when both cells contain exactly those two candidates.
Definition
A naked pair is two cells in one unit whose complete candidate lists contain the same two digits; those digits can be removed from every other cell in that unit.
When to use it and what to verify
When to look for it
Compare complete candidate lists inside one row, column, or 3×3 box after singles stop producing moves.
Required condition
Exactly two cells in the unit must each contain the same complete two-digit candidate list.
Common near miss
A cell containing {3,8,9} does not belong to a naked {3,8} pair. The third candidate breaks the reservation.
R1C2 and R1C5 must use 3 and 8 in some order.
- 1. Match the complete listsR1C2 and R1C5 both contain only {3,8}.
- 2. Reserve the two digitsOne cell will be 3 and the other 8, even though we do not yet know which is which.
- 3. Remove 3 and 8 elsewhereRemove only 3 and 8 from the other cells in row 1. A list {3,8,9} would not be part of this naked pair.
R1C2 and R1C5 consume digits 3 and 8. Candidate 3 or 8 can therefore be removed from R1C1, R1C3, R1C4, R1C6, R1C7, and R1C8.
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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