The "Obvious Pairs" method is basically the same as the "Obvious Singles" method. You have to write the matching candidate numbers in all cells, and then look at the 3x3 blocks that have cells with two identical candidate numbers.
Let's take a look at an example to make things a bit clearer. In the selected block, all the cells are filled with candidate numbers, and there are three cells with repeating digits.
- 2 and 8 (top left);
- 2 and 8 (top right);
- 1, 2 and 8 (bottom left);
The top two cells can only contain 2 or 8, and the other numbers can't (at least, that's what the Sudoku rules say). These are our "obvious pairs." We just need to figure out where the 2 and 8 will go. Then we take a look at the bottom left cell, where we see candidate numbers 1, 2, and 8. We've already figured out that 2 and 8 will be in the top cells, so we delete the numbers 2 and 8 from the bottom left cell, leaving the "Obvious Single"—1. That's what we write in.
One interesting way to solve Sudoku is the "Obvious Pairs" method. It's quick and easy, and it's pretty low-risk.