Yeah, my wife's bike is listed on the frame as 17.5 which I assure you is not cm ...
And tires for MTB's are 2.1, or 2.5. or 3.2, etc. All inches. Dumb I know, when teh metric size is in the small print on the sidewall. I don't know why they are not all just listed as both in the big print? Then everyone would be happy