The Logical One BTG is the rarest version of Romain Gauthier's best-known watch, made in an edition of only five pieces. The 43mm case is titanium with a black coating, finished with a white gold crown and winding pusher. The dial is ink-blue grand feu enamel, fired slightly translucent so the 18k white gold dial plate shows through as the light shifts. The hours and minutes sit on an off-centre subdial beside the small seconds, leaving much of the dial open to the movement, so the mechanism is part of the design.

The Logical One was built to solve one of watchmaking's oldest problems: keeping the force delivered to the escapement constant as the mainspring unwinds. Romain Gauthier's answer is a modern take on the chain and fusee. A snail cam replaces the traditional cone, and a chain with ruby links reduces friction. The mainspring barrel is lined with sapphire to reduce friction further. You wind the watch with a pusher set into the side of the case instead of a traditional crown. The result is steady, consistent power for close to two days, with the 46-hour power reserve shown on the caseback.

The movement is made and hand-finished at the Manufacture Romain Gauthier in the Vallée de Joux. Gauthier trained as a precision engineer before he became a watchmaker, and the Logical One shows that background: it rethinks a centuries-old mechanism and finishes it to the highest traditional standard. Collectors regard it as one of the defining designs of modern independent watchmaking, and with only five made, the BTG is among the hardest versions to find.