A straight-twin engine, also known as an inline-twin, vertical-twin, inline-2, or parallel-twin, is a two-cylinder piston engine whose cylinders are arranged in a line along a common crankshaft.
Straight-twin engines are primarily used in motorcycles; other uses include automobiles, marine vessels, snowmobiles, jet skis, all-terrain vehicles, tractors and ultralight aircraft.
Various different crankshaft configurations have been used for straight-twin engines, with the most common being 360 degrees, 180 degrees and 270 degrees.