In watchmaking, the term conical describes at least three distinct families of technical problems that have almost nothing in common except their shape. Confusing them is a frequent error, even among those who have written about watches for years.
The first family concerns motion transmission across non-parallel planes. A conical gear has teeth arranged on a cone rather than a cylinder: when two conical gears engage on crossed axes, the motion changes direction. In watchmaking this becomes essential whenever the dial is inclined relative to the movement plane. The most discussed case is the MB&F Legacy Machine FlyingT, where the hours-minutes dial is oriented at 50° so it can only be read by the wearer: conical gears are the only way to transmit the couple from the vertical movement to the angled display. The same logic appears in the LMX, the SP One and the Hautlence Sphere Series, where four conical gears on axes crossed at 21° manage the multi-axis rotation of a jumping sphere. Without conical geometry, none of these displays would exist.
The second family is the conical tourbillon. Eric Coudray, working for HYT, revisited an idea by Walter Prendel from 1928 and built a cage in which the balance spiral is inclined at 30° to the horizontal plane, the escape wheel at 15° and the pallet fork at 23°. Complete rotation occurs in 30 seconds rather than 60. The stated objective is to compensate for gravity effects across multiple planes; whether it succeeds better than a traditional tourbillon is an open question, and honestly I don't have sufficient data to answer with certainty.
The third family is purely geometric: bezels, crowns and lugs with a profile that tapers toward the outer edge. On a functional level, a conical bezel offers better finger grip than a flat bezel because the protruding edge acts as a handle. Visually, it emphasizes the perceived thickness of the case. It should not be confused with fluting or knurling, which are surface textures independent of shape.
The conical jewel of the Greubel Forsey Hand Made 2 belongs in its own category: a synthetic ruby cut into a cone, hand-worked over approximately four months, that rotates and rises axially during winding, serving as a power reserve indicator. It is an example of how conical geometry can solve a three-dimensional visualization problem in an extremely limited space.