Withers

Withers are the ridge between the shoulder blades of an animal, typically a quadruped.

[1][2] The withers in horses are formed by the dorsal spinal processes of roughly the 3rd through 11th thoracic vertebrae, which are unusually long in this area.

Generally, a horse should have well-defined withers, as they are considered an important attachment point for the muscles of the torso.

More importantly, the dorsal spinal processes provide an attachment for the muscles that support the shoulder and neck.

If the vertebrae of the withers are long front-to-back, the shoulder is freer to move backwards.