An internal combustion engine configuration.

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'.

Horizonally opposed engines have long been used in light airplanes like the the Piper J3 Cub and Cessna 150/152.

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