N is the size of the subset, which must lie between 2 and the number of unsolved cells in the house minus 2. Since every Naked Subset is complemented by a Hidden Subset, the smallest of both sets will be no larger than 4 in a standard sized Sudoku.
An alternative name for Naked Subset is Number Chain, but this term is not used very often.
Names for Different Sizes
- Naked Pair
- Size 2
- Naked Triple
- Size 3
- Naked Quad
- Size 4
To recognize a Naked Subset, you probably need pencilmarks, unless you have a very good memory. A naked pair is easy to locate because you only need to spot two cells with candidates for the same two digits. A naked triple is more difficult to recognize, because not all three cells need to contain candidates for all three digits. Subsequently, a naked quad is even more difficult to spot.
When all cells are located in an intersection, spotting the subset is much easier. Because there are only 3 cells in an intersection, it is not possible to find Naked Quads in an intersection. The subset can cause eliminations in both intersecting houses. A size 2 is called a Locked Pair and size 3 is a Locked Triple.