High Quality Content by WIKIPEDIA articles! A flat engine is an internal combustion engine with multiple pistons that all move in the horizontal plane. The most popular and significant layout has cylinders arranged in two banks on either side of a single crankshaft, generally known as "boxers". There is a widely-used but technologically less significant form consisting of a straight engine with two, three, four or more cylinders canted 90 degrees into the horizontal plane. The concept of the boxer was patented in 1896 by engineer Karl Benz, eight years after he started producing the world's first successful cars. Flat engines are commonly described as horizontally opposed engines but must not to be confused with opposed-piston engines, which are mechanically quite different.