There may be only one and only one possible choice for a particular Sudoku square.
For example you have a group (row, column or region) that has 8 filled squares
so that leaves only one remaining choice available that must be allocated in that free square.
In our example green cell must have # equal to 7.
5
3
4
6
9
8
6
8
1
9
5
9
1
2
6
8
4
7
6
8
3
2
3
1
6
6
4
1
9
8
2
8
5
7
9
Only square rule
You may find a Sudoku group that has one square in which only one number is possible.
For example if a group has 7 cells with only 2 free numbers.
It is often there is case that a shared group allocated that number and so it must go in the other group.
5
3
4
6
9
8
6
8
1
9
5
9
2
6
8
4
7
6
8
3
2
3
1
6
6
4
1
9
8
2
8
5
7
9
In our example green cells must have # equal to 1 or 7.
Yellow cells help do right guess.
5
3
4
6
9
8
6
7
8
1
9
5
9
1
2
6
8
4
7
6
8
3
2
3
1
6
6
4
1
9
8
2
8
5
7
9
Naked Pairs
The Naked pairs is set of 2 numbers located in 2 cells that belong to the same group.
The solution will contain these values in these two cells and
all other candidates with these numbers can be eliminated from any cells of shared group.
There are two cells with 3-6. There are other cells contain 3 and 7.
We can remove those to generate the right picture.
Naked Triple
Any group of 3 cells in the same group that contain in total 3 candidates is a Naked Triple.
Each cell may have 2 or 3 numbers, as long as in combination all three cells have only three numbers.
When this happens, the three candidates can be removed from all other cells in the same group.
The combinations of candidates for a Naked Triple will be one of the following:
1
abc
abc
abc
2
abc
abc
ab
3
abc
ab
bc
4
ab
bc
ac
We have a triple in columns 1, 8 and 9. There are other squares with 4,6 and 7. These values can be clear off.