A flat engine is an internal combustion engine with horizontally-opposed pistons. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is otherwise known as the boxer, or horizontally-opposed engine. The concept was patented in 1896 by engineer Karl Benz, who called it the "contra engine."A boxer engine should not be confused with the opposed-piston engine, in which each cylinder has two pistons but no cylinder head.