In trying to find a local LBS that could supply me with a specific tire (pair) that I was looking for, I ran into what I believe to be a profound lack of understanding as to tire sizes.
I have a 29er bike that, in many ways, is just an MTB with larger rims. When I explained to the LBS staff and owners what I was looking for, they looked in the 29" section of their catalogs and when we didn't find them, I suggested that they look in the 700c section of the catalog at which point, (1) they questioned whether or not I had a 29er or a road bike and (2) told me that 700c tires wouldn't fit on a 29er rim. I tried to explain to them the ERTO code but all I got were blank expressions (including from the owners of the shops).
I recognize that most 29ers are built like MTB bikes for MTB-style riding so it makes sense that the 29er section of the tire catalog shows wider, knobby tire options but I have found that many popular tires are available in many widths such as the Marathon Plus, from 25-45mm and, for this reason, the LBS catalog may list the 25-32mm widths of this tire in the 700c category and the 35-45mm widths of this tire in the 29er catagory but, nevertheless, they all have the same ERTO code (at least the same 622 component of the code). The worst part of the quest was having an LBS owner tell me that the 622-47 (47mm = 1.85in) tire that I wanted would not fit on 29er rims (but they would sell them to me anyway) because the tires were listed in the 700c category of the catalog. Further compounding the situation was being told that MTB-style tires are measured in inches and 700c tires are measured in mms. I can't think of any road/touring bikes that could use 47mm wide tires and, I'm told that most hybrids top out at about 42mm (or less) so only something like a 29er would have the spacing for this width of tire.
It was frustrating trying to educate the LBS staff.
In the end, I agreed to buy the pair of tires from the LBS with the condition that they cannot be returned if they don't fit: I reluctantly agreed but I almost feel like (when they come in), offering the store a free training session whereby, for the cost (on their part) of a few minutes of their time, they are to install the tire on my front rim (at no cost to me) just so that they learn that 29er rims are the same diameter as 700c rims (not the same width but diameter). I'm not suggesting that I could use a 622-23 tire on my 29er rim, the rim might be too wide for the tire to be properly use. Alternatively, I'm not suggesting that a road bike could use a 622-54 tire because the tire dimensions would not allow it to be fit between the forks/stays/brakes of a road bike. In both cases, however, I believe that the tires could be installed on the "opposing" rims but not necessarily usable.
Please correct me if I'm wrong but I think I'm right about rim size, based on the fact that I took my 54mm Kenda Small Block 8 tires off last year and replaced them with Schwalbe Marathons, 32mm (622-32 or is it 32-622) and was perfectly fine with them. However, for my needs this year, these Schwalbes are too narrow so I've gone with a wider tire, Continental Country Plus, 47mm (622-47).