Sudoku Puzzle Square Possibilities [Page 8 of 15]

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Sudoku Dragon has an unique way of indicating what is possible and what is not possible for any square in the grid.

In this example screen shot the top left region of a 9x9 puzzle is shown. Of the nine squares only four have been allocated. Sudoku Dragon's full possibility analysis has been enabled and so the smaller numbers in the unallocated squares show what are permitted by the rules of Sudoku.

All the different strategies for proving what is and what is not possible are color coded. The light blue background indicates that this is the only possible allocation for one of the groups of nine containing the square. A possibility with a light green background indicates the possibility is the only choice for the square (when other possibilities have been excluded).

Sudoku Dragon uses the exclusion rules that are used to help solve squares, one square in the group has a dark grey background indicating that '4' cannot occur here because of the shared sub-group rule.

This shows Sudoku Dragon's full possibility display options switched on, you can select which if any of these options enabled, here are some of the options available:

Excluded possibilities highlighting switched off.

All highlighting of possibilities switched off.

Highlighting of squares where '4' can occur are switched on.

No automatic possibilities displayed they are left for you to work out

Here's a more detailed explanation of what the colours and numbers all mean. First let us look at the whole grid as that controls a lot of the possibilities in region Aa.

Full puzzle for explanation

Now look at each unallocated square in region Aa.

Square Ab looks like it can take only a 1 or a 2 due to the allocations in the same row; column and region. But in column b a 2 can not go in Cb because of the 2 in Ci nor Gb or Hb due to the 2 in Ha. That means the only square in column b that 2 can go is Ab this is why the 2 is highlighted with a light blue background.
Square Ca looks like it can take only a 4 or a 9 due to the allocations in the same row; column and region. In Row C there is no '4' allocated but looking through the options all the squares Cb;Cc;Cd;Ce and Ch can not take a 4 so it must go in Ca (only square rule again) this is why the 4 is highlighted with a light blue background.
Square Ba looks like it can take only a 4 or a 9 due to the allocations in the same row; column and region. Because square Ca must be a 4 due to the Row C constraints the 4 is not possible here that is why it has 4 with a dark background. Now that 4 is excluded then the 9 is the single possibility left so 9is highlighted as the only choice with a light green background.
Square Cb is another example of the single possibility rule as there is no other possibility other than a 1, as the row; column and region knock out all other 8 numbers. So 1 is highlighted with a light green background.
Square Cc must be a 3 because of both the only choice and only square rules. It is the only square that can take a 3 in column c, Fc and Hc can not have a 3. It also happens to be the single possibility for this square too. Sudoku Dragon in this case hishlights the 3 due to the only square rule.

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