Obvious pairs technique

Strategy #5 for solving Sudoku: Obvious pairs technique

This strategy will help you narrow down the number of assumptions for digits in a 3 by 3 block. 

The secret of this tactic is that if there are two cells in a 3 by 3 square that can accept the same pair of digits, then these digits can only be in those two cells.

In the image above, we see 4 pairs of digits that are candidates for 4 cells. The pairs are 3 and 7, as well as 5 and 9. Since these pairs are within the same 3 by 3 block, we can conclude that the digits 3 and 7 can only be in cells where these candidates exist and nowhere else (highlighted in orange). The numbers 5 and 9, following the same principle, can only be in cells where their candidates are placed.

Based on the above, we can conclude that there is only one cell left that does not contain digits 3, 5, 7, or 9. The only remaining digit is 2, which can be placed in this cell. 

If you liked this technique, be sure to test it on a challenging 9 by 9 sudoku puzzle.