Yes, it's all coming vaguely back to me now.

And the fact that 10 is a triangular number and we use base ten is what gives the number 9 (10-1) its special properties, like if you multiply it by any whole integer above zero and then add the digits of the answer until you get back to one digit again it's always 9. Or something like that. But this is a word forum not a number forum so i'd better shut up now.