The object of su doku is to place the numbers in the squares to follow only one rule: each row, column and 3 x 3 block must contain the digits 1 through 9 exactly once.
Proper su doku puzzles should have only one solution and be solvable by logic alone. You should always be able to determine which number goes in which square without having to guess.
Pencil marks are the little numbers written in a square indicating possible values for that square. You won't be able to go very far in solving su doku puzzles without the use of pencil marks. Su Doku Solver excels at helping you with your pencil marks.
The following nine strategies are some of the more common in use today to solve su doku puzzles. They are listed roughly in order of difficulty. These are the nine that are implemented by Su Doku Solver.