Clifford Periodicity from Finite Groups

We deduce the periodicity 8 for the type of $Pin$ and $Spin$ representations of the orthogonal groups $O(n)$ from simple combinatorial properties of the finite Clifford groups generated by the gamma matrices. We also include the case of arbitrary signature $O(p,q)$. The changes in the type of representation can be seen as a rotation in the complex plane. The essential result is that adding a $(+)$ dimension performs a rotation by $\pi/4$ in the counter clock-wise sense, but for each $(-)$ sign in the metric, the rotation is clockwise.