During the automotive era, there have been inline, vee, round, X and W engines, and one Canadian design with an engine block like a six-shot revolver, but the first practical design to be incorporated in an automobile in the 1880s was flat and it's a configuration that remains in use today.
As the industrial revolution began in the late 1700s inventors were busy trying to create the internal combustion engine, with early efforts involving gunpowder, coal dust, coal (lighting) gas, turpentine and alcohol.
By the 1860s France's Jean-Joseph Etienne Lenoir had developed a workable engine using town gas. And by the mid-1870s German Nikolaus Otto was building two-cycle, town-gas-fuelled, three-metre-tall, single-cylinder engines - big, noisy, inefficient, slow-turning lumps used, as was Lenoir's, to provide industrial power.
By the mid-1870s Otto had developed a more efficient four-cycle engine, and the first one's single-cylinder was laid on its side. Then along came pioneering Germans Gottlieb Daimler, Wilhelm Maybach and Karl Benz.
Benz originally messed about with two-stroke industrial engines, while Daimler worked alongside Otto for the Deutz firm that manufactured his engines. Oddly Daimler's suggestion that petroleum would make an ideal fuel got him fired.
Daimler and Maybach, and separately Benz, were soon creating lightweight, high-revving (up to 900 rpm) engines. Daimler used a tall single, dubbed the "Grandfather Clock" to power the world's first motorcycle in 1885, while Benz employed a horizontal "flat" single to motivate the first three-wheeled car that same year.
Daimler moved on to vee twins, while Benz gave the world its first horizontally opposed style engine in 1897 - later to be known as "boxer" because the pistons punch back and forth. He called it the "contra engine."
The next couple of decades resulted in an amazing level of automotive and motorcycle innovation, but engines evolved mainly along the inline and vee form. But the flat engine didn't go away.
Since Benz's first opposed twin, there have been four-, six-, eight-, 10-, 12- and 16-cylinder boxer engines, powering everything from motorcycles to family transport, sports and racing cars. Plus a wide variety of commercial and military vehicles and aircraft.
Motorcycle designers liked the low centre of gravity they provided and that each cylinder stuck out in the airstream to aid cooling. Over the years Zundapp, Douglas, Wooler, Brough Superior, Ratier, Denpr, Ural, Marusho used flat engines and Honda and BMW still do.
BMW's boxer is easily the most famous and long-lived bike engine, having been created in 1923 and serving as the power source for the company's street, racing and military machines up to the present. Honda revolutionized the touring motorcycle with its first four-cylinder Gold Wing in 1975. It now has six cylinders and more than a million have been built.
Flat-engine-powered cars include models from Volkswagen, Tucker, Chevrolet, Citroen, Jowett, Lancia, Alfa Romeo and, of course, Porsche and Subaru.
Likely the best-known flat car engine is the air-cooled four cylinder that powered Volkswagen's Beetle from the late 1930s until 2003.
Ferdinand Porsche was contracted to build a "people's car" by Hitler in the early 30s, and his design team (with some unacknowledged at the time inspiration from Tatra) came up with the humble, air-cooled, flat-four engined KdF-Wagen. Not many got to the German "Volk" until after the war, but it then went on to become familiar to people around the world.
Porsche's company went on to develop the sports cars that have born that name for more than six decades now, most powered by flat engines.
Modified VW engines were used initially, then Porsche developed its own air- and later water-cooled fours and sixes. It's currently toying with the idea of a new flat four for the "Baby Boxster" due in 2012.
Porsche also created a 1.5-litre flat eight for Formula One racing, used one in its 908 endurance racer, later created a flat 12 and thought about a flat 16 for the 917 racer.
General Motors, which had developed an air-cooled flat six for its Corvair, apparently had a project under way to create a flat 10 for a larger version of the car. Flat 12 engines were designed by Ferrari, Mercedes-Benz and Britain's Coventry Climax built a flat 16 in the 60s, but it never raced.
Citroen's 2CV was France's answer to the VW, a highly engineered, little people mover launched in 1948 that was as ugly as it was inexpensive. It was powered by a 375-cc, air-cooled flat-twin making all of nine horsepower (later boosted to 602 cc and 2 8hp). Because its low cost and toughness, the car stayed in production until 1990.
American Preston Tucker had some novel ideas when he set out to design a new car in the late 1940s, including unique styling and a rear-mounted, air-cooled, six-cylinder flat-six helicopter engine. Only 51 Tuckers were ever made.
Britain's Jowett began making horizontally opposed engined cars in 1910, first with two, then four, cylinders. It survived the war to build the technically advanced Javelin and Jupiter sports car up to 1954.
Subaru introduced its boxer engine in 1966, in its 1000 model, setting it apart from inline-inclined competitors, with all-wheel-drive later adding to its endowments.
The Subie boxer four added displacement and its first rally wins in the 1970s, then overhead cams and turbocharging in the 1980s. An engine could also perhaps be considered "flat" that wasn't horizontally opposed, just disposed on its side, as Toyota did with its mid-engined 1990 Previa minivan.
And if you were wondering about the round, X, W and six-shooter engine configurations mentioned above, Ford experimented with an "X" engine in the 20s. A "W" engine was developed by VW for its Phaeton and Bugatti uses one in the Veyron.
The Monaco-Trossi race car of 1935 used a radial engine with cylinders disposed like the spokes of a wheel. And the Canadian revolver motor, with pistons that fired back and forth like bullets, was created (and ran) in Winnipeg in the early 1980s, but hasn't been heard of since.