A gear system, consisting in basic form of an internally toothed annulus or ring gear and a central externally toothed sun wheel, with usually three or four planet wheels or gears meshing with the sun wheel and annulus. The planet wheels are often carried as a unit on a planet carrier or spider. Holding the rotation of the planet carrier, annulus or sun wheel will provide a transmission ratio between the two remaining elements. Planetary gear sets can be coupled to increase the available number of ratios.