The secret of solving Sudoku
To solve easy sudoku puzzles, all you have to do is to look for unique numbers amongst the candidates for any row, column, or box. However, that technique will not enable you to solve the harder puzzles. For them, you need to apply the rule of pairs, trios, or quartets of candidate numbers. This rule is not easy to state succinctly; but I shall first try to do so for pairs of candidate numbers in any two cells in a row, column or box, as follows:
If a pair of numbers are the only two identical candidate numbers in a pair of cells in any row, column, or box, then those same numbers can be eliminated as candidates from other cells in the same row, column, or box.
Similarly, if there are are only three identical candidate numbers in a trio of cells in any row, column, or box, then those same three numbers can be eliminated from the other cells in that row, column, or box.
Similarly, for only four identical candidate numbers in a quartet of cells.
I hope that is clearer than mud.
Rocker