Acceptable colors of the standard for show purposes are black, brindle, or seal with white markings, seal being a black color with red highlights that are visible mostly in the sunlight. The AKC now recognizes 10 coat colors when registering the Boston Terrier. In the show ring, symmetrical markings are preferred. Size categories are under 15 lbs, 15 to 20 lbs, and 20 to 25 lbs. The breed standard calls for a dog that is short-muzzled and short-backed.
Frequently variations on the standard are seen depending on the ancestry of the individual dog. At various times, the English Bulldog, English Mastiff, Staffordshire Bull Terrier, and French Bulldog—among other breeds—have been crossbred with Boston Terrier lines to minimize inbreeding in what is necessarily a small gene pool.
Originally bred for use in pit fighting, the modern Boston Terrier can be gentle, alert, and well-mannered. They're known to be quite rambunctious (active), with a sense of humor. Both females and males generally bark only when necessary. Having been bred as a companion dog, they enjoy being around people, and if properly socialized get along well with children, other canines, and non-canine pets. Boston Terriers can be very cuddly, while others are more independent. Some Boston Terriers have an excitable or dominant temperament that requires special training and handling by the owner. Some are aggressive towards other dogs and some dogs are just attached to one person.
Several health issues are of concern in the Boston Terrier: cataracts (both juvenile and adult type), cherry eye, luxating patellas, deafness, heart murmur, and allergies.
Curvature of the back, called roaching, might be caused by patella problems with the rear legs, which in turn causes the dog to lean forward onto the forelegs. This might also just be a structural fault with little consequence to the dog.
Many Bostons cannot tolerate excessive heat and humidity due to the shortened muzzle, so hot weather brings the danger of heat exhaustion.