- A Sudoku should have 30 or less initial values filled in out of the 81 total
- Sudoku should be solvable by entirely logical deduction - no guesses should be needed. (That doesn't mean that you can't use guesswork, just that you shouldn't have to.
- The Sudoku must only have one solution!
Of course, for beginner puzzles, often there are more than 30 values filled in - this is just to give you a head start while you're learning!
Many people think that harder Sudoku puzzles are the ones with less values filled in to begin with. While this is often the case, there are plenty of frustratingly difficult puzzles available with 28 or more cells filled in - difficult because they really require you to use lots of different solving techniques to complete them.
Complete the Sudoku puzzle so that each and every row, column, and region contains the numbers 1 to 9 (for 9x9)
1 to 6 (for 6x6) so on..
There is only one solution to a properly designed Sudoku puzzle.