As with other domesticated animals, there are numerous variations. Differences in naming were caused by the different regulations of different kennel clubs. Some dog clubs make a difference between what they call "Parson Jack Russell Terriers", which are over 12" (inches) at the shoulders, and "Jack Russell Terriers" which are under 12" in height. Other clubs do not. They are broadly the same type of dog. It is easy to change the height of dogs by selective breeding, and the reason working dogs come in different sizes is to make them better at a particular job. Taller terriers were used for hunting foxes, and smaller terriers were used for hunting rabbits and rodents.