An internal combustion engine
Cylinders are arranged in pairs in a plane. Each pair lies on the same axis, with the crankshaft perpendicular in the center. The engine is oriented with the piston-crankshaft plane horizontal.
This configuration is notable for low profile, low center of gravity, and low vibration. It has been used in automobiles made by Volkswagen/Porsche and Subaru.
Also called 'boxer', 'pancake', and 'H'.