It would be a lot easier if the industry would stop using 27" or 26" to identify tires and rims and use the ERTRO designations 630, 622, 590, 584 etc., which are based on the bead seat diameter of the tire.

The strangest designation of tire size is the 27 1/2" for 650b ((584mm) mountain bike tires and rims.

The Autumn issue of Bicycle Quarterly has a brief history of tire sizes and how they came to be named.