as I understand it
I seem to recall that "700c," "650a" and "650b" are traditionally the French designations, derived not from the size of the rim, but the outside diameter of the rim plus the appropriate tire. We have always called "650a" "26x1 3/8" in the English-speaking world -- but now that the U.S. isn't really English-speaking anymore it's hard to even find a new bike that is mounted with 26x1 3/8 rims -- the new thing seems to be the 559mm "clunker" size (the so-called "26 inch" relic from the 40's) I thought we buried in the 50's. Apparently the mountain bike brought it back to life (I wouldn't know -- I still don't know why people push those clunkers around). The French "650b" seems to be a non-starter in the rest of the world, despite an effort to import this unneeded size, so close to the "middle-weight" 26x1 3/8 we have always used in the English-speaking world. (Of course, the whole Schwinn "S5" and "S6" middleweight sizes just muddies the water more.) Some see much uniformity and logic in bicycle tire sizes -- but how they do is a mystery to me.