Editing Straight-twin engine (section)

== Terminology ==
The straight-twin layout is also referred to as "parallel-twin", "vertical-twin" and "inline-twin".<ref name="Duckworth2012" /> Some of these terms originally had specific meanings relating to the crankshaft angle or engine orientation; however, they are often also used interchangeably.<ref name="Duckworth2012" /><ref name="Tuttle2005" />

In the United Kingdom, the term "parallel-twin" is traditionally used for engines with a crankshaft angle of 360 degrees, since the two pistons are in the same direction (i.e. parallel to each other).{{citation needed|date=October 2020}} "Vertical-twin" was used to describe engines with a crankshaft angle of 180 degrees,{{citation needed|date=October 2020}} which causes the pistons to travel in opposite directions. The terms "straight-twin" and "inline-twin" were used more generically for any crankshaft angle.

For motorcycles, "inline-twin" has sometimes referred to either a [[longitudinal engine]] orientation (i.e. with the crankshaft in line with the chassis)<ref name="Wilson1995" /><ref name="Henshaw2008" /> or a U-engine ([[U engine#Tandem twin engine|tandem twin]]) where the cylinders are arranged longitudinally in the chassis (although the two crankshafts are actually oriented transversely).<ref name="Walker2000" />