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If you change your mind about a square allocation you can simply undo it, even if the allocation was done a few moves ago. Just use CTRL and the Z key to undo.
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Possibility Explanation [Page 7 of 15])

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You will need to know which numbers can go in particular Sudoku squares. Many other Sudoku puzzle solvers do not give detailed explanations of what can go where and more importantly why.

Sudoku Dragon gives a clear explanation of why certain numbers are not possible according to the Sudoku rules. You select the 'depth' of analysis to determine which strategies will be made use of in making the analysis. As you increase the 'analysis depth' squares marked as 'looking possible' may be excluded to reflect subtler strategy rules. In the case illustrated only the numbers 1 and 4 are possible as all the other possibilities are excluded due to a conflict with a number in a row, column or region.

The more complex twin and shared subgroup exclusion rules can also be used in possibility explanations. Here the exclusion rules have knocked out the possibility of 6; 7 and 9 occurring in this square. The analysis has also spotted that 4 forms one of an alternate possibility pair.

The possibility panel is conveniently located just to the right of the main puzzle display.

Sudoku Possibility Explanation

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