I believe BamaAlumna can answer this better than I can, but here goes...
My understanding of the Machine is that it is comprised of a "select" group of sororities and fraternities... It has been explained to me that not every sorority/fraternity is involved. Also, it was explained that it is mainly those sororities/fraternities that are called "Old Row" that seem to be more involved...
GChatters, feel free to correct me if I am incorrect on any of this info!
