Horse racing is an equestrian performance sport, typically involving two or more horses ridden by jockeys (or sometimes driven without riders) over a set distance for competition. It is one of the most ancient of all sports, as its basic premise – to identify which of two or more horses is the fastest over a set course or distance – has been mostly unchanged since at least classical antiquity. Horse races vary widely in format, and many countries have developed their own particular traditions around the sport. Variations include restricting races to particular breeds, running over obstacles, running over different distances, running on different track surfaces, and running in different gaits. In some races, horses are assigned different weights to carry to reflect differences in ability, a process known as handicapping. While horses are sometimes raced purely for sport, a major part of horse racing's interest and economic importance is in the gambling associated with it, an activity that in 2019 generated a worldwide market worth around US$115 billion. History Horse racing has a long and distinguished history and has been practised in civilisations across the world since ancient times. Archaeological records indicate that horse racing occurred in Ancient Greece, Ancient Rome, Babylon, Syria, and Egypt. It also plays an important part of myth and legend, such as the contest between the steeds of the god Odin and the giant Hrungnir in Norse mythology. Chariot racing was one of the most popular ancient Greek, Roman and Byzantine sports. Both chariot and mounted horse racing were events in the ancient Greek Olympics by 648 BCE and were important in the other Panhellenic Games. It continued although chariot racing was often dangerous to both driver and horse, which frequently suffered serious injury and even death. In the Roman Empire, chariot and mounted horse racing were major industries. From the mid-fifteenth, spring carnival in Rome closed with a horse race. Fifteen to 20 riderless horses, originally imported from the Barbary Coast of North Africa, were set loose to run the length of the Via del Corso, a long, straight city street; their time was about minutes. In later times, Thoroughbred racing became, and remains, popular with aristocrats and royalty of British society, earning it the title "Sport of Kings". Historically, equestrians honed their skills through games and races. Equestrian sports provided entertainment for crowds and displayed the excellent horsemanship needed in battle. Horse