The withers is the highest point on the back of a non-upright animal, on the ridge between its shoulder blades.
They are made up by the dorsal spinal processes of the first 5 to 9 thoracic vertebrae (every horse has 18 thoracic vertebrae), which are unusually long in this area. The processes of the withers are less than 6" in height on the average horse. Since they do not move relative to the ground (as does the horse's head), the height of a horse is measured from the ground to the withers. Horse sizes are extremely variable, from small pony breeds to large draft breeds. The height of the withers on an average Thoroughbred is 16 hands (5' 4").
The withers of the horse are considered in evaluating conformation. Generally, a horse should have well-defined withers, as they are considered an important attachment point for the muscles of the torso. Withers of medium height are preferred, as high withers make it difficult to fit a saddle and are often associated with a narrow chest, and low withers (known as "mutton withers") do not provide a ridge to help keep the saddle in place.
More importantly, the dorsal spinal processes provide an attachment for the muscles that support the shoulder. Horses do not have a clavicle, so the shoulder can freely rotate backwards. If the vertebrae of the withers are long (front to back), the shoulder is more free to move backwards. This allows for an increase of stride length (and so it can increase the horse's speed). It is also important in jumping, as the shoulder must rotate back for the horse to make his forearm more parallel to the ground, which will then raise the animal's knees upward and get the lower legs out of the way. Therefore, the withers have a direct impact on one of the most important points of conformation: the shoulder.
In dogs, the height of the withers is often used to determine the dog's jump height in various dog sports. It is also often a determining factor in whether the dog conforms to the show-quality standards for its breed.
Inflammation of the bursa in this region is called fistulous withers by veterinary surgeons