Normal sudoku rules apply. The grid is covered with fog. Placing a correct digit will clear the fog from the surrounding cells. Cells separated by a knight's move (in chess) cannot contain the same digit. Cells separated by an X contain digits summing to 10. In cages digits must sum to the small clue in the top left corner of the cage. Digits cannot repeat within a cage. A cell with a grey circle must contain an odd digit. A cell with a grey square must contain an even digit. Cells separated by a white dot contain consecutive digits. Cells separated by a black dot contain digits in an 1:2 ratio (ie one digit will be double the other). The ring of 16 cells on the outside of the central 3x3 box (ie touching it either orthogonally or diagonally) – sometimes called the “Phistomefel Ring” – contains digits summing to 109.