Eventing is an equestrian triathlon, in that it combines three different disciplines in one competition set out over one, two, or three days, depending on the length of courses and number of entries.

[citation needed] The dressage phase (held first) consists of an exact sequence of movements ridden in an enclosed arena (20×60 m for International 3DE but usually 20×40 m for ODE).

The challenge is to demonstrate that a supremely fit horse, capable of completing the cross-country phase on time, also has the training to perform in a graceful, relaxed, and precise manner.

Dressage work is the basis of all the other phases and disciplines within the sport of eventing because it develops the strength and balance that allow a horse to go cross-country and show jump competently.

as well as various obstacles such as ponds and streams, ditches, drops and banks, and combinations including several jumping efforts based on objects that would commonly occur in the countryside.

Safety regulations mean that some obstacles are now being built with a "frangible pin system", allowing part or all of the jump to collapse if hit with enough impact.

For every "disobedience" (refusal or run-out of a jump) a horse and rider incur on course, penalties will be added to their dressage score.

The penalties for disobediences on cross-country are weighted severely relative to the other phases of competition to emphasize the importance of courage, endurance, and athleticism.

The 2004 Olympic Summer Games in Athens, Greece chose the short format, due to lack of facilities, time and financing, which sparked a large debate in the eventing community whether to keep the steeplechase phase or just offer cross-country.

In the United States the "classic format" remains a popular option for the Novice, and Training levels of competition at select events.

One change stated that a fall anywhere during the cross-country phase resulted in elimination, even if the rider was galloping on course and not approaching a jump, or in the middle of a combination.

It is a pause designed to allow the horse time to cool off, rest and stabilize its vitals and ensure that it is prepared for the "pure" cross-country phase.

Eventing competition that resembles the current three-day were first held in 1902, at the Championnat du Cheval d'Armes in France, and was introduced into the Olympic Games starting 1912 in Stockholm, Sweden.

Cross-country began as a test of stamina, courage, and bravery over difficult terrain, important for a charger on long marches or if the horse was asked to carry a dispatch across country.

The show jumping phase sought to prove the horse's continuing soundness and fitness after the difficult cross-country day.

Others believe the classic format is superior because it teaches horsemanship, due to the extra preparation needed to condition the horse and the care required after the several miles of endurance day.

However, this claim has not held true in several recent studies that compared injuries sustained in classic and in short format competitions over equivalent courses.

The long format has remained popular at the Novice and Training levels in the United States, and with riders who feel it maximizes horsemanship.

It was quite possible for a horse to have a terrible dressage test, then run a clean cross-country and show jumping, and still finish near the top of the standings.

Since then, correct dressage training has become increasingly important should a horse and rider wish to be placed (complete all sections and finish in the top 12).

A horse can no longer just be brave and athletic but must have a good deal of dressage training should his rider wish to successfully negotiate odd distances or bending lines at a gallop.

Safety measures such as filling in the area between corner-shaped jumps on cross-country or rails of a fence help prevent the entrapment of the legs of the horse decrease the number of serious falls or injuries.

Riders are now required to wear a safety vest (body protector) during cross-country, as well as an ASTM/SEI or ISO approved equestrian helmet equipped with a retention harness,[13][14][15][16] which must be fastened while on the horse.

Andrew Hoy did come close, however, and in 2010 Oliver Townend was competing for this coveted "Grand Slam" at Rolex Kentucky when he suffered a fall at obstacle #20 which eliminated him from competition.

While the 2017 FEI General Assembly proposed the rule change that would implement the new five-star system, there were still plenty of details to iron out.

However, smaller horses can also excel; for example, the third place competitor in the 2007 Rolex Kentucky Three Day CCI competition was Theodore O'Connor, a 14.1 hands (57 inches, 145 cm) gelding that was a cross of Thoroughbred, Arabian and Shetland pony breeding.

For cross-country, the horse must be brave, athletic, and (especially at the higher levels) fast with a good galloping stride and great stamina.

Cross-country attire and equipment emphasizes and requires safety protocols be followed, but has less formal appearance, with many riders wearing clothing of personalized, often bright colors.

At the upper levels, riders usually have a saddle specifically designed for cross-country, giving them more freedom for such fences as banks and drops.

Other forms of equipment, such as martingales, protective boots, gadgets/training devices, bit guards, exercise bandages, or tail wraps, are not allowed during the test.