Books of Sudoku puzzles are very common, and typically categorize the Sudoku into Easy, Difficult, and Very Difficult. There is no clear standard for classification, and the Very Difficult puzzles of some books can be solved by relatively elementary techniques. However, all Sudoku require that the solver make no mistakes, or the puzzle becomes a mess and can be difficult to clean up.
The instructions found in these books all point out that one should never simply guess, except with very difficult Sudoku where one may choose one of two options, to solve by some variation of Ariadne's thread.
