Sudoku Puzzle Square Possibilities [Page 8 of 14])

Sudoku Dragon has a unique way of indicating what is possible and what is not possible for squares in the puzzle.

In this 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 options have been selected and so the smaller numbers in the unallocated squares show what are permitted by the rules of Sudoku.

Color coding is used to show which strategy could be used to solve a square. 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 gray background indicating that 4 cannot occur here because of the shared sub-group rule.

You have full control on how the possibility analysis is displayed. The previous example shows it with the full display options switched on, but you can choose other options including:

Excluded possibilities highlighting switched off.

All highlighting of possibilities switched off.

Highlighting of squares where a 4 can occur.

No automatic possibilities displayed, you need to work them out yourself.

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

Full puzzle for explanation

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 preceding analysis on Row C a 4 is not possible in Ba that is why it has 4 with a dark background. Now that 4 is excluded then the 9 is the single possibility left so 9 is 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 as 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 highlights the 3 due to the only square rule.

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