Hell, 700c is nominal tire diameter.

700C is a rim+tire combo that's supposed to be 700mm across. Do the math, that's a 39mm tire if you really had a 700C tire. Yowza.

29ers are supposed to be about 29" with a 2.3" tire.

27" calls for a 1-1/4" tire.

26" means 1.75" tires.