The peripheral rotor solves a problem that the central rotor has never ceased to pose: the oscillating mass mounted at the center of the plate adds thickness and obscures the mechanism. Moving that mass to the perimeter of the movement changes everything, geometrically and optically.
The principle is straightforward: the oscillating weight takes on an annular or curved segment shape and rotates along the outer circumference of the plate instead of turning on a central pivot. The center of the movement remains free. Axial thickness is reduced because the rotor does not overlap the bridges: it sits alongside the edge of the plate, occupying the same radial plane as the movement instead of adding a layer on top. It's the difference between an open book and a book with a coin glued to its cover.
From the workbench, I've observed three distinct families. The classical central rotor, which I never ask a customer to appreciate aesthetically: it simply covers the movement. The micro-rotor, which frees almost the entire view but has reduced mass and charges more slowly. The peripheral rotor, which combines more generous mass—and therefore more efficient winding—with total visibility of the mechanism. It is not a universal solution: it requires guides or rolling elements along the inner edge of the movement, constructive solutions that add complexity and cost. Jaeger-LeCoultre in the caliber 362 uses 36 ceramic spheres. Bulgari in the BVL 388 achieves 3.50 mm total movement thickness. Vacheron Constantin in caliber 2160 mounts the rotor in 22-carat gold and achieves 5.65 mm overall height while leaving the tourbillon cage completely visible from the caseback.
Industrial paternity of the system belongs to Carl F. Bucherer, who began mass-producing it from 2009 onward, bringing to maturity a concept that had circulated in technical literature since the 1950s without finding systematic application. Before then, the peripheral rotor remained more of a stylistic exercise than a repeatable solution at scale.
An interesting constructive variant, which I saw on the CODE41 NB24, positions the peripheral rotor on the side of the bezel rather than on the caseback. It is rare because it implies a complete rethinking of the movement's architecture, not simply a relocation of the component.
In haute horlogerie, the peripheral rotor is today one of the most reliable indicators that a caliber has been designed in-house, not assembled from standard components. It is not an absolute guarantee of quality, but it is a technical constraint that filters out those who haven't truly invested in proprietary development.