It's size, of course, particularly the width of the pillar plate. "The SIZE as a unit has its basis in the old "0" size movement. This movement measures 1 5/30". A difference of 1/30" accounts for each size smaller or larger than the starting point of the system, the "0" size watch movement."Genejockey, love your collection and history. Can you educate me a bit further. I'm not sure I fully comprehend what is meant by 8/0, etc. I've read the definition/description of how this corresponds to size, but I'm not sure I fully grasp it.