Originally Posted by
teachme
Is that where the 700 comes from? The measure of diameter in millimeters?
Yes, millimeters, the C is just a designator of the rim width, as already pointed out there used to other widths named A, B, and D with A being narrowest and D the widest, C was the most commonly used so they just went with C's as the standard 700 mm rim, discontinuing the others for the most part. They are also called 29 inch rims :
https://en.wikipedia.org/wiki/29er_%28bicycle%29