Small-wheel bicycles are adult bicycles that have wheels of 510 mm (20 in) nominal diameter or less, which is smaller than the 700c (522 mm), 660, 700, 740 mm (26, 27.5, 29 in) sizes common on most full-sized adult bikes. While many folding bicycles are small-wheel bicycles, not all small-wheel bicycles can fold. Some small-wheel bicycles neither fold nor separate, such as the Moulton, which comes in both fixed-frame and separable-frame versions. While BMX bikes also have 510 mm (20 in) wheels, they are not normally categorised as "small-wheel bikes".[1]