In the beginning, there was the single-cylinder motorcycle engine, and it was good. Early motorcycles, like all vehicles in the late 19th century, were crude-essentially bicycles with unreliable, oil-dripping, flame-belching internal-combustion powerplants often scarcely stronger than a man. In other words, they were a perfect match for America's highway system, which wasn't much of a system at all. But everything improves, and eventually the need for more power (certainly more reliability) than a single-cylinder engine could provide won out on the desire for utmost mechanical simplicity. After all, a single is about as simple as it gets, but running it hard may result in a less reliable engine than a more complex design that's loafing along. If you're already making one cylinder, it's not too much of a stretch to imagine plonking another on a single crankshaft-bingo, double the displacement and, so the theory goes, double the power. In fact, done right there are some great advantages to the V-twin; for instance, one cam can actuate valves in two different cylinders or a single carb could share two intake manifolds. Strictly speaking, you wouldn't necessarily be doubling the parts count for a useful boost in (still, at this point, theoretical) power.