What is the difference between a Jack Russell and a Parson Russell?

The differences between a Jack Russell and a Parson Russell are minimal since the two breeds were not considered separate until the late 1990s and early 2000s by three major kennel clubs — the United Kennel Club (UKC), Australian National Kennel Club (ANKC), and the New Zealand Kennel Club (NZKC). The American Kennel Club (AKC) does not recognize the Jack Russell and Parson Russell as distinct breeds; instead, it currently only recognizes the Parson Russell. 

The two breeds have similar physical traits, but there are some key differences. Jacks tend to have narrower chests, shorter legs, and more rectangular, elongated bodies than the stockier, taller, and more square build of Parsons.