For example say you got an E_8 X E_8 heterotic string theory and you compactify it on a Calibi Yau with SU(3) holonomy it breaks one of the chiralities down to an E_6 G.U.T. So you have something like E_6 X E_8. What do you do with that other copy of E_8? How is that interpreted? I don't know much about this subject, but that just seems kind of weird.