Kropki Sudoku puzzles add black (o) and white (o) circles to a regular Sudoku puzzle. All pairs of touching squares which contain consecutive values (such as 2 & 3 or 6 & 7) are marked with a white circle: o. Also, all pairs of touching squares which contain values where one number is exactly twice the value of the other (such as 2 & 4, or 4 & 8) are marked with a black circle: o. This applies to left/right, up/down touching only - there are no diagonal markers. All possible o and o are given, so if there is no circle between two touching squares then they are not consecutive and one is not double the value of the other.