There are two sizes of arenas: small and standard. Each has letters assigned to positions around the arena for dressage tests to specify where movements are to be performed.

The small arena is 20 m by 40 m (66×131 ft), and is used for the lower levels of three-day eventing in the dressage phase, as well as for the USDF Introductory tests and the USEF Training Level tests. Its letters around the outside edge, starting from the point of entry and moving clockwise, are A-K-E-H-C-M-B-F. Letters also mark locations in the middle of the arena: Moving down the center line, they are D-X-G, with X in the center. Since the combination of Equine Canada (EC) and United States Dressage Federation (USDF) tests in 2003, the small size arena is no longer utilized in rated shows in North America.
Standard dressage arena, 20 m by 60 m (66×197 ft).

The standard arena is 20 m by 60 m (66×197 ft), and is used for tests in both dressage (USEF First Level and above) and eventing. The standard dressage arena letters are A-K-V-E-S-H-C-M-R-B-P-F. (There is speculation as to why these letters were chosen. Most commonly it is believed because the German cavalry had a 20 x 60 meter area in between the barracks which had the letters posted above the doors) The letters on the long sides of the arena, nearest the corners, are 6 m (19.7 ft) in from the corners, and are 12 m (39.4 ft) apart from each other. The letters in the middle of the arena are D-L-X-I-G, with X marking the center.

At the start of the test, the horse enters at A. There is always a judge sitting at C, although for upper-level competition, there are up to five judges at different places around the arena—at C, E, B, M, and H—which allows the horse to be seen in each movement from all angles. This helps prevent certain faults from going unnoticed, which may be difficult for a judge to see from only one area of the arena. For example, the horse’s straightness going across the diagonal may be assessed by judges at M and H. Judges in the United States are licensed by the USEF for different levels of competition, depending on the judge’s experience and training.

The dressage arena also has a centerline (from A to C, going through X in the middle), as well as two quarter-lines (halfway between the centerline and long sides of each arena).

Dressage  (a French term, most commonly translated to mean “training”) is a path and destination of competitive horse training, with competitions held at all levels from amateur to the Olympics. Its fundamental purpose is to develop, through standardized progressive training methods, a horse’s natural athletic ability and willingness to perform, thereby maximizing its potential as a riding horse. At the peak of a dressage horse’s gymnastic development, it can smoothly respond to a skilled rider’s minimal aids by performing the requested movement while remaining relaxed and appearing effortless.  Dressage is occasionally referred to as “Horse Ballet” . Although the discipline has ancient roots, dressage was first recognized as an important equestrian pursuit in the West during the Renaissance. The great European riding masters of that period developed a sequential training system that has changed little since then. Classical dressage is still considered the basis of trained modern dressage.

Early European aristocrats displayed their horses’ training in equestrian pageants, but in modern dressage competition, successful training at the various levels is demonstrated through the performance of “tests” of prescribed series of movements within a standard arena. Judges evaluate each movement on the basis of an objective standard appropriate to the level of the test and assign each movement a score from zero to ten – zero being “not executed” and 10 being “excellent”. A score of 9 is considered “very good” and is considered a particularly high mark, while a competitor achieving all 6s (or 60% overall) should be considering moving on to the next level.

All riding horses can benefit from use of dressage principles and training techniques. However, horse breeds most often seen at the Olympics and other international FEI competitions are in the warmblood horse breeds category. Dressage is an egalitarian competition in which all breeds are given an opportunity to compete successfully. Therefore, many other breeds are seen at various levels of competition.