Abstract: A gear mechanism, for example for controlling the rotation of vehicle rear-view mirrors, comprises a first sun gear (1) driving a first set of planet gears (5, 6, 7, 8) which mesh with a first ring gear (9), a second set of planet gears (5, 6, 7, 8) of different diameter to the gears of the first set, the gears in the second set meshing with a second ring gear (10) but not with the first sun gear, the first and second ring gears being located side-by-side and relatively rotatable, wherein the each planet gear in the first set of planet gears is rigidly connected to a respective one of the planet gears in the second set so as to be rotatable therewith. Various devices for detecting the relative angle between a tractor and trailer vehicle so as to control the steering of a mirror are also disclosed, as well as means for zeroing the mirror position automatically.