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Naked One

The Naked One method is the first of the two basic methods. Although the approach may seem trivial, Naked Ones are often harder to see than you might expect.

Each field is belongs to three logical units. One row, one column and one block. The field initially contains all allowed candidates (typically the candidates 1 to 9).
Now you remove from this list all candidates that have already been used in the row, column and block. Then, if only one candidate is left in the list, you have found a "Naked One".
Since this is the only remaining candidate, it can now be entered in the field.

In this image only the 5 can be entered in the green field because all other numbers have already been used somewhere (row/column/block).

The Naked One is a special case of the Naked group.

Sudoku example for naked One

see also: Generalization: Naked groups



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