A flat-twelve engine, also known as a horizontally opposed-twelve, is a twelve-cylinder piston engine with six cylinders on each side of a central crankshaft.

A V12 engine is a twelve-cylinder piston engine where two banks of six cylinders are arranged in a V configuration around a common crankshaft. V12 engines are more common than V10 engines. However, they are less common than V8 engines.