He is an active terrier of medium bone, strongly put together, suggesting endurance and agility, but rather narrow in shoulder, body and quarter. The body is covered with a somewhat broken though close-fitting and intensely wiry jacket. The characteristic “otter” head with its keen eye, combined with a body poise which is “at the alert,” gives a look of fearless and implacable determination characteristic of the breed. Since the Border Terrier is a working terrier of a size to go to ground and able, within reason, to follow a horse, his conformation should be such that he be ideally built to do his job. No deviations from this ideal conformation should be permitted, which would impair his usefulness in running his quarry to earth and in bolting it therefrom. For this work he must be alert, active and agile, and capable of squeezing through narrow apertures and rapidly traversing any kind of terrain. His head, “like that of an otter,” is distinctive, and his temperament ideally exemplifies that of a terrier.