The Border Collie is a British breed of herding dog of medium size. It originates in the region of the Anglo-Scottish border, and descends from the traditional sheepdogs once found all over the British Isles. It is kept mostly as a working sheep-herding dog or as a companion animal. It competes with success in sheepdog trials. It has been claimed that it is the "most intelligent" breed of dog.
Border Collies are, on average, medium-sized dogs with a moderate amount of coat, which is often thick and prone to shedding. They have a double coat that varies from smooth to rough and is occasionally curled. While black and white is the most common colouring of the Border Collie, the breed appears in just about any colour and pattern known to occur in dogs. Some of these include black tricolour (black/tan/white), liver and white, and red tricolour (red/tan/white) which have also been seen regularly, and other colours such as blue, lilac, red merle, blue merle, brindle, and Australian red (also known as ee red, blonde, recessive red, or gold) which is seen less frequently. Some border collies may also have single-colour coats.
Eye colour varies from brown to green, and occasionally eyes of differing colour occur; this is usually seen with merles. This trait is known as heterochromia. The ears of the border collie are also variable — some have fully erect ears, some fully dropped ears, and others semi-erect ears (similar to those of the rough collie). Although working border collie handlers sometimes have superstitions about the appearance of their dogs (handlers may avoid mostly white dogs due to the unfounded idea that sheep will not respect a white or almost all white dog), in general, the American Border Collie Association considers a dog's appearance to be irrelevant. Instead, it is considered more useful to identify a working border collie by its attitude and ability.
Dogs bred for show are more homogeneous in appearance than working border collies since to win in conformation showing they must conform closely to breed club standards that are specific on many points of the structure, coat, and colour.