Solve Sudoku

One additional tool to help solve sudoku puzzles is to look at a square that has no candidates or perhaps just one. Take a square with no candidates in it and have a look for numbers that can NOT be considered a candidate. Use cross hatching and candidate pairs within a box. If you have found candidate pairs in a box other than the square you are looking, then it is impossible for that number to exist in that suare. By a process of elimination you may be able to find the only number that could exist in that square.