Molecular planetary gear, computer model. Each of the coloured spheres represents a single atom. A planetary gear consists of a train of interconnected gears. A central (or sun) gear (red tube) meshes with several planet gears (grey and black), which mesh in turn with an outer (or ring) gear (large yellow tube). This arrangement allows the output and input gears to share the same axis. The gear ratio can also be varied by changing which gears form the input and output, and which are held still. This device is a hypothetical future component for nanotechnology (devices constructed at the molecular level). Designed by K. Eric Drexler and Ralph Merkle.