Complex problems can often be solved by finding ways to split them into several smaller problems. In a Sudoku, each house is a problem that has 362880 possible solutions (the possible permutations for the 9 digits). To reduce this number, a player will try to isolate groups of cells and digits from the remainder of the house.

When N cells and N digits are isolated, a subset of size N is formed.

Alternative terms are disjoint subset, locked set and number chain.

Solving Sudoku Strategy - Subsets

Naked Pair 2 cells with candidates for 2 digits.

Naked Triple 3 cells with candidates for 3 digits.

Naked Quad 4 cells with candidates for 4 digits.

Hidden Pair 2 digits with candidates in 2 cells.

Hidden Triple 3 digits with candidates in 3 cells.

Hidden Quad 4 digits with candidates in 4 cells.

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