A flat-twin engine is a two-cylinder internal combustion engine with the cylinders on opposite sides of the crankshaft. The most common type of flat-twin engine is the boxer-twin engine, where both pistons move inwards and outwards at the same time.
Douglas 80 Plus motorcycle engine (circa 1950)
Citroën 2CV engine (viewed from rear)
1912 Douglas N3 engine
1942 Harley-Davidson XA engine
A flat engine is a piston engine where the cylinders are located on either side of a central crankshaft. Flat engines are also known as horizontally opposed engines, however this is distinct from the less common opposed-piston engine design, whereby each cylinder has two pistons sharing a central combustion chamber.
Benz Contramotor, the first commercial flat-engine design, ca. 1899
1954 BMW R68 flat-twin boxer engine
World War II-era Riedel starter motor
1969 Hino Motors DS140 boxer-twelve diesel engine