Intermediate
Naked pair
Two cells in a unit hold the same two-digit candidate set.
When two cells in a row, column, or box have been pared down to the same two candidates — call them {3, 8} — those two digits are committed to those two cells. Which one ends up where doesn't matter; the rest of the unit is locked out of both.
It's the entry point to subset reasoning. Once the pattern is in your hands, triples and quads work the same way at higher arity.
When the move applies
Naked pairs surface as soon as your pencil marks are clean. Any two cells in the same unit that show the same two candidates and nothing else are doing this.
The procedure
- Find two cells in one unit whose candidate sets are identical and contain exactly two digits.
- Strike those two digits from every other cell in the unit.