Orientations
The terms longitudinal engine and transverse engine are most often used to refer to the crankshaft orientation, however, some sources, most prominently Moto Guzzi, use the terminology in the opposite way.
A Moto Guzzi Technical Services representative tried to explain to LA Times columnist Susan Carpenter that Moto Guzzi engines are "called 'transverse' because the engine is mounted with the crankshaft oriented front to back instead of left to right." In spite of this, it is could be assumed that those who call V-twin motorcycle engines "transverse" when they are mounted with the crankshaft front-to-back and the cylinders sticking out the sides are saying that to them, the engine's axis is the line passing from one cylinder to the other, at a right angle to the crankshaft, rather than going by the crankshaft's axis. Highly technical sources, such as V. Cossleter's Motorcycle Dynamics, or Gaetaeno Cocco's Motorcycle Design and Technology are careful not simply to use the terms "longitudinal engine" or "transverse engine," but rather to specify that they mark the engine's orientation based on the crankshaft, and so they will say "transverse crankshaft engine" or "longitudinal crankshaft engine", or, conversely, "transversely mounted cylinders" in referenced to the classic BMW orientation, with a longitudinal crankshaft and cylinders at a right angle to the axis of the frame.
Read more about this topic: V-twin Engine