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...)
The way this term is used, it's mostly a way of categorizing the bike. 20 inch is kid size BMX bikes or the kind of thing you'd buy at Toys R Us with a banana seat, but not the super small kids' bikes, which probably have something in the neighborhood of 14 inch wheels. Adult sized mountain bikes have 26 inch wheels.