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The withers is the ridge between the shoulder blades of a four-legged mammal. In many species it is the tallest point of the body, and in horses and dogs it is the standard place to measure the animal's height (in contrast, cattle are normally measured to the top of the hips).
The withers in horses are formed by the dorsal spinal processes of roughly the 3rd through 11th thoracic vertebrae (most horses have 18 thoracic vertebrae), which are unusually long in this area. The processes at the withers can be more than 12 inches (30 cm) long.
Since they do not move relative to the ground (as the horse's head does), the withers are used as the measuring point for the height of a horse. Horses are commonly measured in hands – one hand is 4 inches (10.16 cm). Horse heights are extremely variable, from small pony breeds to large draft breeds. The height at the withers of an average Thoroughbred is 16 hands (64 inches, 163 cm), and ponies are up to 14.2 hands (58 inches, 147 cm)
Conformational issues 
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 and neck. 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.