Tic
A tic is a sudden, repetitive, nonrhythmic motor movement or vocalization involving discrete muscle groups. Tics can be invisible to the observer, such as abdominal tensing or toe crunching. Common motor and phonic tics are, respectively, eye blinking and throat clearing.
List of motorcycles by type of engine
The Tandem Twin is where the cylinders are longitudinal, and have two cranks geared together. A tandem twin is effectively a pair of geared singles, and is to be distinguished from an inline twin. Tandem twins are suitable primarily for two-stroke racers.