There is, of course, a significance to the numbering system. In the Orient, three is the most esteemed of all the numbers. Note: The Chinese character for "3" contains three lines: the upper line is to symbolize heaven, the middle line represents mortals and th elower line the Earth.
Knowing the significance of the number 3 explains more clearly the structure behind the ranks. The degree grades are spilt into three groups of three. First through Third degree are considered the novicce stages of black belt, whilt fourth through sixth are considered expert grades. Once the third set is reached (seventh to ninth dan) the practicioner is considered a master.
The colour ranks (or kup) are also broken into groups of three:
The first group (9th through 7th Kup) covers all belts with Yellow in them
The second group (6th through 4th Kup) cover the dark hues of belt
The final group (3rd through 1st) covers all belts with Red in them