Bianchet B1.618

B1.618 is a line from the independent Swiss brand Bianchet, named after the golden ratio, 1.618. This ratio and the Fibonacci spiral determine the proportions of the tonneau case and, in the watches described here, the architecture of the movement as well.

The watches are powered by the skeletonised manufacture calibre B1.618: hand-wound, with a flying tourbillon in the lower part and bridges in Grade 5 titanium. There is no closed dial — the movement can be seen straight through, and from the back via the transparent caseback. The case is a 43 mm carbon tonneau on a rubber strap.

The versions share the calibre, case size and water resistance, and differ in colour, case materials and rarity: the Carbon Red Tourbillon Openwork Limited Edition has red accents, the Carbon Tourbillon Openwork Unique Piece green ones. The manufacturer's B1.618 family is wider still, including the Grande Date and Sport GMT lines. The brand's other collection is Bianchet Tourbillon.