Working on the principle that the more different positions the regulating organ moves through, the more precise the timekeeping will be, Robert Greubel and Stephen Forsey invented the Quadruple Tourbillon with a spherical differential. Right from the original idea the concept was clear. While coupling four separate tourbillons would have been challenging, they sought a more compact solution. To save space in all three dimensions, they chose to construct the tourbillons in pairs within a unique system of compact cages, an extremely challenging feat. Each of these two systems is modelled on the Double Tourbillon 30°, with a first cage rotating in one minute and angled at 30°, fitted inside a second upright cage performing a full rotation in four minutes.
Having mastered the tourbillon, Robert Greubel and Stephen Forsey have continued to investigate and reinterpret other horological mechanisms, notably a simultaneous multiple time zones display. In 2011, the two Inventor Watchmakers expressed their own vision of this modern day function in the ground breaking timepiece aptly named the GMT, more recently joined by the GMT Earth. In this patented system, the second time zone indication on a separate subdial is paired with a universal time display providing an intuitive reading of the time around the world thanks to a world first: an outsized globe completing one full rotation every 24 hours, following planet Earth itself.
Universal time is visible through the case back, with a fixed 24-hour scale bearing day/night zones and a disk with three-letter abbreviations of 24 cities representing the various time zones. This disk also distinguishes between the time zones that implement Summer Time (Daylight Saving Time) appearing in a light colour; and those that do not (shown on a dark backdrop). The case back also provides a chance to admire the Quadruple Tourbillon as well as frosted bridges with jewels set in gold chatons. The two complex asymmetrical convex sapphire crystals protecting the dial and case back also called for the utmost expertise.