The index is a component, often made of steel, located on the balance bridge. It is on the same axis as the balance wheel, but unlike the balance wheel, it is on the upperside of the bridge and not on the inside. The index regulates the active length of the balance-spring and thus the rate of the watch.

The index is a component that can greatly stand out in the artisanal method. It can be bevelled on the top and bottom. Its top surface can be black polished. Its internal diameter can be adjusted by hand without splitting.

The index can also be manufactured using the semi-artisanal method. For this method, it is mainly produced by electro-erosion and sometimes by stamping. It will be summarily decorated. The diameter can be adjusted more easily using a split diameter.

The index is a component whose manufacture can be simplified for the industrial method. Its decoration will be more basic. Its internal diameter will not be adjusted but will be split.