Skeletonizing a movement means removing material from the mainplate and bridges until only the structural elements strictly necessary for mechanical function remain. The result is not decorative in the conventional sense: it is a design choice that requires whoever executes it to understand the movement deeply, because every extra millimeter removed compromises the rigidity of the assembly, every millimeter left unremoved makes the work pointless. Armin Strom built his reputation on this, even before founding his manufacture.
The technique applies to different components with different results. When working on the mainplate and bridges, we speak of a fully skeletonized movement: the gear train, the mainspring, the regulating organ and the escapement become legible from both sides. When instead intervening only on the dial — selectively removing the base material and opening windows onto the mechanics — you obtain a partial skeleton, which preserves time readability while sacrificing less rigidity. The Speake Marin Openworked Dual Time uses this second approach: the rhodium-plated bridges and the tungsten micro-rotor remain visible without the dial losing structure. The Zenith El Primero 3620 SK goes almost all the way, exposing the caliber almost entirely through the front surface.
The term openworked is the Anglo-Saxon equivalent used by the official communication of many manufactures, but in the workshop the two words describe the same operation. The difference that matters is another: between skeletonization executed by hand, file after file, and that produced by CNC milling. The first leaves surfaces that light treats unpredictably; the second is more uniform and recognizable.
On the automatic rotor, skeletonization has a precise mechanical rationale beyond the aesthetic: lightening the oscillating mass reduces inertia and, in certain use profiles, improves charging response. The tungsten micro-rotor is an alternative solution that achieves a similar result in terms of visual footprint, concentrating mass in a smaller diameter thanks to the material's density.
The degree of finish on the surfaces exposed by skeletonization is the true test. Anglage, satin finishing, mirror polishing on the cut edges: on a closed movement no one sees them. On a skeletonized one they are the first thing you read. This is why skeletonized movements from manufactures require decoration times disproportionate to those of mechanical processing.