  We point out that the choice of phases in GSO projections can be accounted
for by a choice of fermionic SPT phases on the worldsheet of the string. This
point of view not only easily explains why there are essentially two type II
theories, but also predicts that there are unoriented type 0 theories labeled
by n mod 8, and that there is an essentially unique choice of the type I
worldsheet theory. We also discuss the relationship between this point of view
and the K-theoretic classification of D-branes.
