One of the most common and popular techniques to solve Sudoku puzzles is by using pencil marks. In complex Sudoku puzzles, it is helpful and necessary to use pencil marks for a hit and trial method. For example, if you’re certain that a particular box can be filled by one of only two numbers, you can use a pencil to add one of the numbers and see if it works out. If it doesn’t, you can simply erase it and replace it with the only other remaining number option to complete the row/column/3x3 grid accordingly.