Tire sizing is very confusing. The best explanation is here on Sheldon Brown's site, though it doesn't make a complex system simple, because it can't.
Briefly, even Europe used inches to describe tire sizes. The stupidity started when the size referred to the outside diameter of the inflated tire rather than the rim size. On a given rim, you can switch tires and get a much different diameter, so that's really dumb. Then eventually, the sizes we had referred to what USED TO BE the size of the tire.
700c and 29er both refer to rims with a bead seat diameter (BSD) of 622mm. That's weird because 700c is slightly smaller than 27" wheels with have a BSD of 630mm. And Europeans refer to 700c as 28", which, again, is smaller in actual size than 27".
In England, there was a so-called 28" size whose BSD is 735mm, only slightly larger than 27" (630mm), and 5mm is nowhere near an inch.
700mm is kinda-sorta close to 27", so we have 700c, but 700a, 700b, and 700d are never used as designations.
650mm is kinda-sorta close to 26", and there are lots of 26" tires, some of which use a 650 designation, and this does not help! 650a has a BSD of 590mm, and this is what English 3-speed and many other bikes use. It's often called 26x1-3/8". Some other sizes are also called 26x1-3/8" which is enough to drive people mad. 650b is 584mm. 650c (with 571mm BSD) came into style a few years ago as narrow-tired racing bikes for shorter people. It was a good idea, but bike makers found it too expensive to have different sizes, so now shorter people have to go back to racing bikes with 700c wheels, even though the fit is not optimal. Then there is the MTB style 26" which is much smaller than all of the 650 designations, as the BSD is 559mm.
Also, some inch sizes are expressed with decimal fractions and some with simple fractions. Do not interchange these with numeric equivalents, because types of fractions IMPLIES the BSD. So you can have 26x1.375" but that is not 26x1-3/8.
Are you crazy yet?
