Single In Box

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Single In Box
When there is only one cell in the box which can contain a certain value, this value can be safely assigned to this cell. This is the most common technique used to solve a puzzle.

In the example below every known value 8 is marked blue. The red lines indicate some of the cells that cannot be 8 because there already is an 8 in the row or column. Therefore, the yellow marked cell must be 8 because this is the only possible 8 in the box.

Single