What are Double and Quadruple Tourbillons?

– Double Tourbillons refer to two tourbillons in the same watch regulating 2 separate escapements. Gruebel Forsey developed a unique double tourbillon mechanism with the first tourbillon angled at 30 degrees rotating inside of a second tourbillon that made a revolution every 4 minutes. The concept of a Quadruple Tourbillon was also first released by Greubel Forsey. This used two double tourbillons working independently with a spherical differential connecting the two sets of tourbillon mechanisms.