CHARACTERISTICS. The terrier must present a lively, active and alert appearance. It   should impress with its fearless and happy disposition. It should be remembered   that the Jack Russell is a working terrier and should retain these instincts.   Nervousness, cowardice and over-aggression should be discouraged, and it   should always appear confident.
GENERAL APPEARANCE. A sturdy, tough terrier, very much on its toes all  the time, measuring between 10 and 15 inches; at the withers.
The body  length must be in proportion to the height, and it should present a compact,  balanced image, always being in solid, hard condition.
HEAD. Should be well balanced and in proportion to the body. The skull  should be flat, of moderate width at the ears, narrowing to the eyes. There   should be a defined stop but not over-pronounced. The length of muzzle from  the nose to the stop should be slightly shorter than the distance from the   stop to the occiput. The nose should be black. The jaw should be powerful and  well boned with strongly muscled cheeks.
EYES. Should be almond shaped, dark in colour and full of life and  intelligence.
EARS. Small Vshaped drop ears carried forward close to the head and of moderate thickness.
MOUTH. Strong teeth with the top slightly overlapping the lower.
NECK. Clean and muscular, of good length, gradually widening at the shoulders.
FOREQUARTERS. The shoulders should be sloping and well laid back, fine at points  and clearly cut at the withers.
Forelegs should be strong and straight boned   with joints in correct alignment.
Elbows hanging perpendicular to the body  and working free of the sides.
BODY. The chest should be shallow, narrow and the front legs set not too widely apart, giving an athletic, rather than heavily chested appearance.
As a guide only, the chest should be small enough to be easily spanned behind   the shoulders, by average hands, when the terrier is in a fit, working condition. 
The back should be strong, straight and, in comparison to the height of the  terrier, give a balanced image.
The loin should be slightly arched.
HINDQUARTERS. Should be strong and muscular, well put together with good  angulations and hand of stifle, giving plenty of drive and propulsion.
Looking   from behind, the hocks must be straight.
FEET. Round, hard-padded, of cat-like appearance, neither turning in nor out.
TAIL. Should be set rather high, carried gaily and in proportion to body   length, usually about four inches long, providing a good hand-hold.
COAT. Smooth, without being so sparse as not to provide a certain amount   of protection from the elements and undergrowth. Rough or broken coated, without being woolly.
COLOUR. White should predominate with tan, black, or brown markings. Brindle  markings are unacceptable.
GAIT. Movement should be free, lively, well   co-ordinated with straight action in front and behind.
PLEASE NOTE:
   1) Dogs and bitches should be entire and capable of breeding. Dogs should be shown to have both testicles fully descended into the scrotum.

   2) Old scars or injuries, the result of work or accident, should not be  allowed to prejudice the terrier's chance in the show ring unless they   interfere with its movement or with its utility for work or stud.

   3) For showing purposes, terriers are classified into two groups according to their height, which are 10 to 12½ and over 12½ to 15 inches
Correct General Appearance
   (Illustration courtesy of JRTCA)
A Jack Russell Terrier should not show any strong characteristics of  another breed
The
JRTC of Great Britain
Original
Breed Standard
U12= 10-12,5 Inches   &   O12= 12,5-15 Inches
25,4 - 38 cm