Dee Rings or Egg Butt Rings?

What's the difference between dee rings and eggbutt rings?
Both options have rings attached to the mouthpiece, and the reins’ contact will be more still towards the mouth. These rings will encourage a horse to keep the contact towards the bit and create a stable feeling for both the horse and rider. (How much contact and stability you get depends on what mouthpiece you choose to go with it.) So, what is the difference then?