EXTRACT FROM: The Allen Illustrated Guide to Bits and Bitting

The shape and length of the head, thickness of the lips, depth of the mouth, thickness of the tongue. The height of the roof, position of the curb groove and width of the face, are all things that have great importance as to how each bit is going to work, and fit, and how comfortable your horse is going to be. It is very important to check your horses face and mouth on a regular basis, so that you are alerted to any injury or abnormality instantly, and to know the mouths inner structure.

Thick tongue: the size of the tongue has great significance on how each bit will work, the action of most bits is divided between the tongue and the bars of the mouth. With a thick tongue the bit will touch the bars much later and with a thin tongue much earlier.

To check to determine just how thick the tongue is, part your horse's lips [with the teeth still closed] does the tongue appear to fill the mouth cavity? And fill or even bulge out over the bars? Indicating that perhaps there isn't that much room for a very thick bit or perhaps a combination of bits.

In this situation you must think of creating more room, a single jointed snaffle might be changed for a French link Snaffle, or a Medium ported Snaffle. A Show Horse or Cob for instance instead of a Double Bridle might wear a Rugby Pelham with a medium port allowing the tongue more room but still creating the illusion of a Double Bridle, the floating "Bradoon" ring can be attached to a Bradoon head piece. With a Dressage horse, possibly a French link Bradoon with a medium port Weymouth. In short with thought you can within reason follow the needs of your chosen sport and still ensure your horse is comfortable.