Due to their working nature, Jack Russell terriers remain much as they were some 200 years ago.[33] They are sturdy, tough, and tenacious, measuring between 10–15 inches (25–38 cm) at the withers,[34] and weigh 14–18 pounds (6.4–8.2 kg).[15] The body length must be in proportion to the height, and the dog should present a compact, balanced image. Predominantly white in coloration (more than 51%) with black and/or tan markings,[34] they exhibit either a smooth, rough or a combination of both which is known as a broken coat. A broken-coated dog may have longer hair on the tail or face than that which is seen on a smooth-coated dog.[35] The skin can sometimes show a pattern of small black or brown spots, referred to as "ticking" that do not carry through to the outer coat.[15] All coat types should be dense double coats that are neither silky (in the case of smooth coats) nor woolly (in the case of rough coats).
The head should be of moderate width at the ears, narrowing to the eyes, and slightly flat between the ears. There should be a defined but not overpronounced stop at the end of the muzzle where it meets the head, and a black nose. The jaw should be powerful and well boned with a scissor bite and straight teeth. The eyes are almond shaped and dark colored, and should be full of life and intelligence. Small V-shaped ears of moderate thickness are carried forward on the head.[15] When the dog is alert, the tip of the V should not extend past the outer corner of the eyes. The tail is set high and in the past was docked to approximately five inches in order to provide a sufficient hand-hold for gripping the terrier.
The Jack Russell should always appear balanced and alert.[34] As it is primarily a working terrier, its most important physical characteristic is its chest size, which must not be so large that it prevents the dog from entering and working in burrows.[15] The red fox is the traditional quarry of the Jack Russell terrier, so the working Jack Russell must be small enough to pursue it. Red foxes vary in size, but across the world they average from 13–17 pounds (5.9–7.7 kg) in weight and have an average chest size of 12–14 inches (30–36 cm) at the widest part.[36]
Differences with related breeds
The Parson Russell terrier (pictured) shares a common ancestry with the Jack Russell Terrier.
The Jack Russell Terrier and Parson Russell terrier breeds are similar as they share a common origin, but with several marked differences—the most notable being the range of acceptable heights.[37] Other differences in the Parson can include a longer head and larger chest as well as overall a larger body size.[38] The height of a Parson Russell at the withers according to the breed standard is 12–14 inches (30–36 cm) which places it within the range of the Jack Russell Terrier Club of America's standard size for a Jack Russell of 10–15 inches (25–38 cm). However the Parson Russell is a conformation show standard whereas the Jack Russell standard is a more general working standard.
The Russell terrier, which is also sometimes called the English Jack Russell terrier or the Short Jack Russell terrier is a generally smaller related breed.[40] Both the breed standards of the American Russell Terrier Club and the English Jack Russell Terrier Club Alliance states that at the withers it should be an ideal height of 8–12 inches (20–30 cm).[41][42] Although sometimes called the English or Irish Jack Russell terrier,[43] this is not the recognised height of Jack Russells in the United Kingdom. According to the Jack Russell Club of Great Britain's breed standard, it is the same size as the standard for Jack Russells in the USA, 10–15 inches (25–38 cm).[44] Compared to the Parson, the Russell terrier should always be longer than tall at the withers, whereas the Parson's points should be of equal distance.[45] The Fédération Cynologique Internationale standard for the Jack Russell terrier has this smaller size listed as a requirement.[46] Terrierman Eddie Chapman, who has hunted in Devon for more than 30 years, the same area that John Russell himself hunted, notes that, "I can state categorically that if given the choice, ninety-nine percent of hunt terrier men would buy an under 12" worker, if it was available, over a 14" one.
