The modern American Quarter Horse has a small, short, refined head with a straight profile, and a strong, well-muscled body, featuring a broad chest and powerful hindquarters. They usually stand 14-16 hands high, although some may grow as tall as 17 hands.
   There are two main body types: the stock type and the racing type. The stock horse type is shorter, more compact, stocky and well muscled, yet agile. The racing Quarter Horse is built to sprint short distances ranging from 220 - 870 yards, and therefore is somewhat taller and smoother muscled than the stock type, more closely resembling the Thoroughbred.
   The American Quarter Horse is a breed of horse originally bred for sprinting short distances, typically races of a quarter mile or less. The Quarter Horse is the most popular breed in the United States, and is the largest breed society in the world, with over 3.2 million Quarter Horses registered worldwide. It is commonly believed to be the world's fastest horse over short distances, and has been clocked at 55 mph.