Eventing is an Olympic equestrian sport consisting of three phases, historically held over three days. The rider must ride the same horse for all phases, a true test of rideability, courage, and carefulness. A format change in the early 2000s made the event shorter, now commonly held on one day and referred to as a “horse trial” at the lower levels. Eventing and horse trials are used interchangeably.

The three phases of an Event or Horse Trial are:
1. Dressage
2. Cross country
3. Show jumping

The dressage phase must be first, the cross country and show jumping phases may follow in any order.

Combined Test:
The combined test consists of a dressage phase and a show jumping phase, in any order. A horse and rider pair counts as one entry, with combined dressage and jumping scores determining the outcome.

Derby
Commonly, derbies include only one phase: a jumping phase that includes cross country and show jumping elements. However, derbies may also include a dressage phase. Combined scores for horse and rider pair determine the outcome.

Find more information about the sport:

Discover Eventing – and informative site offered by USEA. Note some information is specific to USA competitions.

Wikipedia – Eventing