Two-cylinder motorcycles are called "twins." The three most common arrangements are

The "V-twin" where the cylinders form a "V" around the crankshaft, which is oriented transversely (i.e., perpendicular to the direction of travel).[2]
The inline twin or straight-two, which is common in classic British motorcycles and Japanese motorcycles. It is known as a parallel twin when the cylinders share a common crank pin. In this design the cylinders are side by side vertically above the crankcase. If not vertical they are generally nearly so in order to maximize airflow cooling.

The opposed twin in which the cylinders protrude sideways into the cooling air stream.
The angle in the V-twins varies from around 45 degrees to 90 degrees. Typical of the former are the Harley-Davidson and Vincent engines which because of their firing order tend to vibrate more. Ducati and Moto Guzzi make V-twins with cylinders arranged at a 90 degree angle to quell primary vibrations. Some Moto Guzzi motorcycles have V-twins oriented transversely: one cylinder to the left, one to the right.

The parallel twin engine configuration was made famous by Edward Turner's Triumph Speed Twin design as used on the Triumph Bonneville.

In the BMW[3] flat-twin ("boxer twin") engine, and used as well now by the Ural[4] and historically by Douglas[5], the cylinders are horizontally opposed, protruding from either side of the frame. The boxer is the only twin-cylinder arrangement that has inherent primary balance without a rocking couple, producing very low vibration levels without the use of counterbalance shafts.

Sunbeam produced an air cooled inline twin driving a propshaft.

Narrow-angle V-twin engines dominate the cruiser motorcycle segment.
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