Straight-twin engine

1964 Sanglas Rovena motorcycle engine (built by Hispano Villiers)
1997 Suzuki GS500 motorcycle engine

A straight-twin engine, also known as an inline-twin, vertical-twin, or parallel-twin is a two-cylinder piston engine where two 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, jetskis, 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.