The Dee-ring, as its name suggest, has a ring shape like a "D" with the cheek side of the "D" attached to the mouthpiece of the bit. The straight bars of the Dee-ring provide a slight lateral guiding effect. This is because the bit ring is pulled against the side of the mouth opposite the rein that is activated, pushing the sides of the Dee against the horse's mouth, encouraging a turn. The Dee-ring is fixed in the horse's mouth, because its shape does not allow the bit to rotate. The Dee-ring is most similar to the full cheek.
Sweet Iron - will rust when in contact with humidity (air). The surface rust tastes sweet and naturally simulates the salvation production. This will cause the hors to salivate more which will encourage the horse to take more contact.
The process of rusting (oxidisation) changes the blue colour of the Trust bit to a brownish - grey. A sweet iron bit will develop an orange-brow rust layer on the mouthpiece if it is temporarily not used. This is easily removed by wiping it off with a damp cloth.