Skeletonized and open-worked are not perfect synonyms, even though many catalogues use them interchangeably. The technical distinction exists and is worth maintaining: a skeletonized movement removes material from the plate and bridges until only the strictly necessary load-bearing structures remain, making the movement legible from both sides. An open-worked piece may limit itself to a partially pierced dial—a hole at six o'clock to show a tourbillon, for example—without the plate being touched. In other words: every skeletonized watch is open-worked, but the reverse is not true.
The skeletonization process is one of the most demanding operations at the bench. You start with a raw plate and remove all non-structural material using milling cutters or electrical discharge machining. What remains must withstand the dynamic stresses of the movement without deforming. Remove too much and the plate flexes, throwing the gear train out of alignment. Remove too little and the visual effect is mediocre, failing to justify the cost. The margin for error is narrow. Surface finishing of the internal geometry—hand chamfering, Côtes de Genève, anglage—happens after skeletonization, on shapes no machine can easily reach, and that is where the true level of a manufacture reveals itself.
The same principle applies to individual components. A skeletonized rotor is an automatic rotor with material removed: it reduces oscillating mass and allows you to see the movement through the sapphire caseback. A skeletonized date wheel is simply a lightened wheel, often blued for decorative reasons. Even lugs can be skeletonized—material removed to lighten the case or create a visual suspension effect, as in 3D-printed constructions.
The term semi-skeletonized describes a dial that exposes only part of the underlying mechanism, retaining enough structure for legibility. It is a compromise solution common in mid-range watchmaking, where full skeletonization would entail decoration costs difficult to justify at the final price point.
A special case is the inverted movement: rather than skeletonizing the plate, you flip the calibre so that the decorated side—normally hidden—becomes the dial. The visual result is similar, but the constructive logic is opposite. It is not skeletonization; it is architectural rotation.
At the bench I have seen skeletonization done well and skeletonization done poorly. The difference is immediately apparent: sharp edges, residual burrs, matte surfaces where there should be shine. Skeletonization forgives no approximations because every millimetre of remaining metal is in full light.