When we talk about rims we usually speak of their dimension in inch. A 24-inch rim, a 20-inch rim, a 28-inch rim...., however, this is more a "commercial dimension" and is rather not precise and may lead to misunderstandings and mistakes. The rim dimension tells us if the rim is big or small, furthermore it tell us which tires can be mounted. The dimension in inch of a unicycle wheel tells us the approx. final diameter of the wheel including the tire correctly mounted and with the right pressure. It progressively led us to indicate with the same dimension in inch (i.e. 26-inch), rims and tires having different diameters, but all with a very similar final diameter of the complete wheel. Of course, if we try to mount on a rim a tire not of the right dimension, it will be not possible. To overcome these problems the European Tire Rim Technical Organisation (ETRTO) has adopted a measurement in millimeters and has indicated how this measure must be clearly identified so that it is possible to define rims and tires compatible with each other. The Etrto classification for the rims tells us the diameter in "mm". We will find the same measurement on the tire, which is referring to the width of the tire. An Etrto measurement will appear like this: 37-622 (mm), where 37 indicates the transversal diameter of the tire (the width) and 622 indicates the referring diameter of the rim. There is also a French standard, which indicates the external diameter of the cover in mm, and its width with a letter at the end. This letter tells us the inside diameter of the cover and the rim. A French measurement will appear like this: 700x28C (mm), where 700 indicates the external diameter of the cover, 28 its width, and the "C" the internal diameter (in this case 622).