Hidden Pairs, Trio's and Quads

This is an excellent way to reduce the number of possible solutions. The method is very easy

Start by looking for unique identical pairs which appear only twice in a row, column or box. This means that the pair of numbers you have found have to be on one of cells, so all other number can be eliminated.

First Example :

solve sudoku by finding hidden pairs

In the example above, I have found the number pair (3,8) in the first row in the cells H1 and I1.

Because the numbers 3 and 8 do not appear anywhere else in this row, the only solution for the cells H1 and I1 is either a number 3 or a number 8. This means that all other solutions can be eliminated.

After elimination the content of cell H1 will be (3,8) and for cell I1 also (3,8).

Second Example :

solve sudoku by finding hidden pairs

In the cells C3 and C4 the number pair (1,4) has been found. This number pair is unique for the box, so all number except 1 and 4 can be eliminated in the cells C3 and C4. The same can be done for the cells G5 and G5 with (3,7) as unique number pair.

Triples and quads

Instead of looking for number pairs you can also look for triples and quads. Quads however appear seldom in Sudoku puzzles.