The Breastcollar/ Polo Breastplate
The breastcollar consists of a chest strap, which buckles to one billet of the saddle, runs around the horse's chest, and attaches to the first billet on the other side. It also has a wither strap, which is used to adjust the height of the breastcollar, and prevents it from slipping down too far. The breastcollar is often made of leather, strong elastic, or webbing.
The breastcollar is more secure than the hunting breastplate, because it attaches to either the front billet of the saddle, or to the front branch of a split-end girth (which is even more secure). It is therefore most desirable in eventing, especially on the cross-country phase, polo, and other jumping disciplines. It is not used in dressage, hunt seat, or equitation.
This style of breastcollar does not interfere with the horse's shoulders, as some other styles can do. However, this style may interfere with the horse's ability to breathe when it puts its head far down. Thus, this style is not desirable for jumping and riding in steep terrain.
Read more about this topic: Breastplate (tack)