A twin boundary or composition surface separates the two crystals. Crystallographers classify twinned crystals by a number of twin laws. These twin laws are specific to the crystal system. The type of twinning can be a diagnostic tool in mineral identification.
The industry responded by making middleweight tires, marked 26 x 1.75 to fit the same rims. Although they are still called 26 inch, these tires are actually 25 5/8, not 26. This same rim size was adopted by the early pioneers of west-coast klunkers, and became the standard for mountain bikes. Due to the appetite of the market, you can get tires as narrow as 25 mm to fit these rims, so you wind up with a 26 inch tire that is more like 24 7/8 in actual diameter! A second number or letter code would indicate the width of the tire. (26 x 1.75, 27 x 1 1/4...650B, 700C...)
Most road bikes and touring bikes have thinner tires, while mountain bikes have big fat tires. Each type of tire has been adapted for the surfaces they ride on.The road tires are inflated to 100 or even 120 PSI (pounds per square inch).