The index-assembly gathers all the peripheral components to the regulating organ (balance-spring) and allows for the correction of its adjustment. Depending on the construction and chosen adjustment method, the number of its components may vary. For instance, you generally won’t find an index-assembly in a movement with a terminal curve hairspring (such as Breguet or Philips).

The index-assembly, depending on its construction, may include one or more of these components:

The first components associated with what we now call “index-assembly” logically appear with the invention by Christian Huygens of the balance-spring as a regulator in 1675. However, the first watches regulated in this way did not truly have an index. The first appearance of this component is credited to John Harrison in his famous H4 chronometer in 1759. Nonetheless, Harrison’s index was not designed to allow for an average correction of the regulation by statically modifying the active length of the spiral, as it is done today. Already aware of the influence of thermal variations, Harrison designed and placed the index in his H4 to thermally compensate for the expansions and contractions of the spiral. To achieve this, he capitalized on his work on bimetallic constructions (which had already earned him the invention of the bimetallic thermo-compensating balance).

Harrison’s index was articulated on a double blade composed of two different metals. Through their combination, it tilted to the left when temperatures were dropping and to the right when they rose. Since the movements of the index were opposite to those of the spiral (expansion-contraction), the index corrected the active length of the spiral during temperature variations.

Although very complex to master, this system did not experience widespread adoption and quickly lost its appeal as material mastery and adjustment theories progressed. However, the index has been present in the majority of watches since then and continues to play a role in the static adjustment of the watch’s regulation.