A system devised and patented by Abraham-Louis Breguet in 1801 to compensate for errors of rate caused by the Earth's gravitational force in upright positions. The escapement is mounted in a revolving cage with the regulating organ (balance) at the centre.
The escape-wheel pinion turns about the fixed fourth wheel. The cage generally revolves once a minute and, in doing so, compensates for errors of rate caused by the vertical position in which pocket watches spend most of their time.
This delicate and complex structure is one of watchmaking's most ingenious mechanisms. A simplified and more robust alternative is the karussel where the cage is driven not by the fourth wheel but by the third wheel. A tourbillon can contain a lever or a detent escapement.