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Single In Group

The most basic technique in Sudoku is the "Single in Group" technique. In Sudoku, each row, column and box must have all the digits from 1 to 9 in it. So, if you have a group (a row, column or box) with 8 digits already present, there is only one possibility for the last digit. This case is shown below:

In the above example, the cells highlighted in orange have all been filled, leaving only one possibility for the cell highlighted in red: it must be an "8". Any other choice would leave two instances of the same digit in the box, and no "8". Another example is shown below:

In this case, the orange highlighted column is missing a "1", which must go in the cell highlighted in red.

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