Double bridles are most commonly associated with dressage and certain horse show classes where formal tack, attire and turnout is standard. They are required for upper level FEI dressage tests.
A Double Bridle is a bridle that has two bits and four reins. The bridoon is a modified snaffle bit that is smaller in diameter and has smaller bit rings than a traditional snaffle. The Weymouth is a curb bit with shanks and a curb chain.
The Bridoon is adjusted so that it sits above and behind the Weymouth, and the two are sized to compliment each other.