Advanced Sudoku Solving Techniques

If you get stuck using the basic methods, then there are other tools at our disposal. The first is to take a look at a square that has no candidates in it. Starting with the number 1 see if it already occurs in either the column, the row or as a candidate pair. If it occurs elsewhere in the block as a candidate pair then it cannot occur in the square that we are looking at. Also if the number occurs as a candidate pair anywhere in the column or row then it cannot be in the square, but the candidate pair must be aligned with the target square. Work your way through all nine numbers and if there is only one candidate left then it is safe to assume that it is the answer.