A dee ring snaffle is a cross between an eggbutt snaffle and a fulmer snaffle.
It is less bulky than an eggbutt, prevents pinching at the corners of the mouth, and has more control as it has longer cheeks/shanks than an eggbutt.
Because of this degree of control and safety, this bit is mostly used in show jumping and racing.
The dee shanks are longer and straighter than an eggbutts, so this bit works more on the sides of the mouth, giving the rider more control with steering, without the bit being able to be pulled through the mouth.
The curved mouthpiece creates more room for the tongue, making it more comfortable for the horse.