"Epicyclic gearing" or "planetary gearing" is a gear system consisting of one or more outer, or "planet", gears or pinions, revolving about a central "sun gear" or "sun wheel". Typically, the planet gears are mounted on a movable arm or "carrier", which itself may rotate relative to the sun gear. Epicyclic gearing systems also incorporate the use of an outer "ring" gear or "annulus", which meshes with the planet gears. Planetary gears (or epicyclic gears) are typically classified as simple or compound planetary gears. Simple planetary gears have one sun, one ring, one carrier, and one planet set. Compound planetary gears involve one or more of the following three types of structures: meshed-planet (there are at least two more planets in mesh with each other in each planet train), stepped-planet (there exists a shaft connection between two planets in each planet