I don't exactly know what "the principle of ten" is, but by the way Eharasan explained it, the numbers are not a way of measurement. I mean not as in 1 less than 9 type deal.
In order to get to 10, 1 is a compliment of 9, and 9 is a compliment of 1, and so on:
_________
1&9  9&1
2&8  8&2
3&7  7&3
4&6  6&4
5&5  5&5

Remember trigonometry? Complimentary angles and supplementary angles.
