Originally Posted by herzogone

As alan s referenced earlier (and just elaborated on), details can be found at http://sheldonbrown.com/tire-sizing.html. To summarize, 700C was from French sizing, originally based on the nominal diameter in mm, including a specific size tire (C was a width designation, there were also A, B, and D). It also notes that 29" is just a marketing term for some wide 700C tires (mostly MTB, presumably based on the approximate outsize diameter of the tire). As most have mentioned, ISO is the most clear, it consists of two numbers, the inner rim width in mm, and the bead seat diameter of the rim. When in doubt, check ISO size.