Ann, as the numbers go up the shutter gets bigger. If you think about lens/aperture characteristics in very basic terms, as focal lengths get longer the maximum aperture needs more space. 10" lens at f10 will need a 1" hole for the light to go through. f5 would need a 2" hole. So longer lenses to keep a good size aperture for focusing need bigger and bigger shutters. In years gone by different company's had size 0 1 2 3 4 5, and one company's 3 was different than the next company's 3. An Ilex 3, Compound 3, and Copal 3 are all different sizes. In fact Amercan made shutters like Ilex all had american threads in inches while Compund (German Compur) and Copal had metric. Finally by the mid '70's things had settled down to where there are for the most part just three sizes left, and all pretty much interchangeable. Copal 0, 1, 3 The 0 has a thread about 29mm wide. The 1 has a thread about 40mm and the 3 has a thread about 58mm.