Car engines have almost always come with even numbers of cylinders. Four, six, eight, 10; even the occasional two-cylinder.
So it seemed pretty crazy when - not all that many years ago - Audi introduced a five-cylinder engine.
Engineers had known differently for years but, to the general public, having an engine with an odd number of cylinders raised eyebrows. Wouldn't it be all out of balance?
This scepticism was despite the existence of a few three-cylinder cars, ones that tended to be not well known outside Europe. The Saab 93 "inverted spoon" design of 1955 was a prime example - all the quirkier because it was also a two-stroke, like an old motor mower.
Nowadays, five-cylinder vehicles are nearly mainstream; although many come from Europe, Japan and America have also joined in.
But why? Surely a five isn't going to be that much better than a four, and "less good" than a six? The advantage of a five-cylinder motor is deeply technical and centres around its power stroke, the characteristics of which allow the engine to run more smoothly than a four-cylinder engine, although a balancer shaft will still be needed.
A six- or eight-cylinder engine will usually be smoother than a five, but the extra cylinders bring with them more friction, more parts and more weight. The five is highly regarded for its overall efficiency - it's a great compromise. The straight-five configuration also presents a size compromise between a straight-four and a straight-six that's useful for engineers designing cars with a transverse engine, when reasonable cubic capacity and power are required.
Factory accountants like the fact that a five can be developed from "modular" four- or six-cylinder inline engines, saving cost and eliminating the need for a different production line.
And a five-cylinder engine provides an important point of marketing difference, something not to be under-estimated in the quest to attract showroom traffic.
For example, General Motors claims the five-cylinder has the power of a V6, and the fuel economy of a four-cylinder, although neither statement is strictly true. European manufacturers tend to push the "technology" of the five-cylinder engine.
Volkswagen has its own twist, the 2.3-litre V5. Actually, it's not a true Vee, with two cylinders on one side and three on the other, but a staggered straight bank. It has its origins with the company's acclaimed VR6 engine, hence its designation, VR5.
Among manufacturers to use, or which have used, five cylinder engines in cars, light commercials or SUVs are Acura (the Honda luxury badge), Audi, Fiat, Ford, General Motors (including Hummer), Mercedes-Benz, Saab, Toyota, Volkswagen and Volvo.
Honda used a V5 in its RC211V MotoGP race bike and the configuration is used in some trucks including Scania. So if a five-cylinder car turns up on your car-shopping shortlist, don't write it off as an oddball. It's a sensible choice with genuine benefits.
Staying with the "five" theme, some vehicles have engines with five valves per cylinder. This allows the cylinders to be "ventilated" more efficiently, providing more torque and power along with lower fuel consumption and emissions. Audi was the first carmaker to mass-produce five-valve engines.