Here is a graphical illustration of the tourbillon parts. The tourbillon device has the escape wheel, lever, and balance wheel all mounted in a carriage made of a lightweight material (usually titanium). The carriage turns 360 degrees at regular intervals (usually once per minute). The fourth wheel (not shown) is fixed and is concentric with the carriage pinion and arbor. The escape wheel pinion meshes with the fourth wheel and rotates around the fourth wheel in the fashion of a satellite. The escape wheel and lever are mounted on the carriage, and the third wheel (not shown) drives the carriage pinion, turning the carriage once every minute. This rotation of the escapement helps reduce the positional errors of a watch.