Flat engines

Flat engines:

A flat engine is an internal combustion engine with multiple pistons that move in a horizontal plane. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is sometimes known as the boxer, or horizontally opposed engine.

Flat engines offer a low center of gravity and thereby may offer a drive configuration with better stability and control. Flat engines lend themselves well to aircraft engines.