# Why are unorientable strings with reversed orientations different?

If a string is unorientable, why is the a string with reversed orientation different from the initial string? Why do we have Kalb-Ramond 2-forms?

-

Among the 5 ten-dimensional superstring theories, only type I string theory has unorientable strings. In modern language, type I is type IIB with a spacetime-filling orientifold O9-plane, which makes the strings unorientable, and 16 spacetime-filling D9-branes and their images which allow open (and equally unorientable) strings in the spectrum and give them the $SO(32)$ gauge group carried by the end points.