Sudoku can often be solved just using the techniques outlined in part I of this strategy guide. However, some sudokus - for instance fiendish ones - often require you to use a technique called deriving certainty from uncertainty. Here is an example:
Look at this grid:
We want to know where the '2' goes in box one. It looks like it can go in three places: top right, top middle, and middle right. We don't yet know where the two goes in the top middle square to help us out. However, we can place it in one of two squares: top left or top right. Since we now know that the '2' in the second box must be in the top left or top right squares, we can use this to rule out the top right and top middle squares in box one for the location of the '2'.
This is because, of course, there can only be one of each number in a row - therefore if there has to be a two in the top row in the middle square, it cannot be in the top row of the first square. This only leaves one square for the '2' in the top left square - the middle right, and we place the two here:
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