The withers are located above and just behind the horse’s shoulders where you would find the shoulder blades. They are the long thoracic vertebrae (roughly the 3rd through 11th) that support his head and neck.
They are an important attachment point for the muscles of the torso. The shoulder rotates freely backward, using the muscles attached to the withers. If the vertebrae of the withers are long, the shoulders have more room to rotate and the stride of the horse is long. This is very important for speed or jumping horses.
Withers that are too high make it difficult to fit a saddle and are often associated with a narrow chest. Withers that are too broad (called “mutton withers”) do not provide enough ridge to keep the saddle in place on the horse’s back. (see Saddles)
Since they do not move relative to the ground (as his head does) withers are used to measure a horse. Measure from the highest point of the withers to the ground. Horses are measured in “hands” (each hand is 4 inches). Thoroughbreds are usually about 16 hands. Ponies up to 14.2 hands but not taller.