Normal sudoku rules apply. Nine copycat cells exist in the grid. They all contain the same digit (to be deduced), but they all have a different value. You can use the circles at the bottom to keep track. The value of a copycat cell equals the digit in the cell that is 180° rotationally opposite itself. Eg: If r1c3 is a copycat cell, then its value equals the digit in r9c7. For two adjacent values along a grey line, one can always be divided by the other to give an exact integer - ie; one is a multiple of the other. If a black dot sits between two cells, the value of one is double the value of the other. If a yellow dot sits between two cells, their values are not consecutive or the same.