Tznius for women: Where does the neck end?
The poskim say that the neck ends where the person’s body (torso) starts. In the front it is at the collar bone. Even in the center, where there is a “v” shape, that is also considered the neck. In the back it is until the first vertebrae of the spine. (This bone can be felt sticking out when one bend their head forward). On the side, it is tricky, the part of the neck that is vertical is obviously not the body, however when the neck angles towards the shoulders, and it is hard to determine. The poskim say that it is where a necklace will lay naturally.
Oz Vehadar Levusha pg. 263.