Part of the regulating assortment which oscillates to and fro in sync from the power of the balance spring. Lengthening and shortening of the balance spring make the balance wheel go faster or slower to advance or retard the watch.
There are two primary types of balance wheel: index regulated and free sprung balance. Although some have both.
The more premium type of balance wheel “free sprung” does not need an index to be regulated, instead, it uses adjustable weights, and balance poising to be adjusted for precision (this is considered more premium compared to an index regulated balance). Some balance wheels have both, which is rare.