Normal sudoku rules apply. A binary code is a sequence of numbers (“bits”) that are either on or off. In this puzzle, Odd digits represent bits that are 'OFF'(O) and Even digits represent bits that are 'ON'(E). This gives us the following representation of the digits 1-9: 1 → OOOE 2 → OOEO 3 → OOEE 4 → OEOO 5 → OEOE 6 → OEEO 7 → OEEE 8 → EOOO 9 → EOOE The digit in an arrow cell dictates the parity (odd or even) of its four orthogonal neighbours. These four neighbours must match the binary code of the digit with the small arrow. The arrow points to the first bit of the binary code. (read from left to right.) The full code must wrap around the central cell clockwise. Eg If a 6 is placed in an arrow cell, its code is OEEO. Therefore, its four neighbours (starting from the arrow direction, moving clockwise) must be Odd, Even, Even, Odd.