This photo shows the part of an english riding saddle known as the "billets". The billets are three straps underneath the top flap, on each side. The billets are the anchor to which the girth is tied. The billet straps are securely attached to the tree (what could be called the "skeleton" of the saddle) so once a girth is buckled to the billets it fits the saddle to the conformation of the horse's back and prevents the saddle from slipping (if the girth is sufficiently tightened).
Billet Strap Safety
Billet straps are the most vunerable part of the girthing mechanism so it is important to inspect your billet straps regularly. If billet straps are still and brittle instead of supple and strong, a sharp intake of the horse breath or sudden expansion of the chest (like during jumping or descending into a river crossing) can cause the billet straps to break and the saddle to come off! Check your billet straps regularly for signs of cracking or stiffness. Generally billet straps should not be oiled during regular tack cleaning, as oil can cause them to stretch, but a light coat of conditioner may be applied to dry, stiff billet straps.
Why are there 3 billet straps, but 2 buckles on a girth?
Even though there are three billet straps, the english girth is designed only to attach to two of them. This is in case one billet strap breaks in the field or your horse is not comformationally comfortable with the girth where it naturally sits. Some horse, and some saddles, require that the girth be buckled to the front two billet straps, or the back two, but for the vast majority of horses, the girth should be buckled to the outer billets, as this provides the most stability for the saddle.
