Here is a list of tire sizes.
Tire Sizing Systems

In the past, tire sizes were the approximate outer diameter of the tire, and not the rim. So different tire widths would have different sizes. There were several 26" sizes.

EXCEPT 27" which was generally standardized.

Now, there are several dominant rim sizes, generally marked with metric fractional sizes, so a 29er MTB tire will fit on a 700c road rim, if the rim is wide enough.

Perhaps 24" tires are the only ones that haven't quite been standardized yet between "Road" and "MTB/BMX" versions, although I think there are a couple of different 650 tires out there???