Different bits employ pressure on different parts of the horse's head in order to achieve the desired result. The places where bits can act are:

1. the tongue
2. the bars
3. the lips
4. the roof of the mouth
5. the chin groove
6. the nose
7. the poll
8. the side of the face

In general, thinner bits are harsher by applying pressure over a reduced area, while fatter bits are milder by applying pressure over a wider area, however some small-mouthed or thick-tongued horses are uncomfortable with a thick bit, as they do not have enough room in their mouth for it.

Loose ring bits are supposed to give more "play" in the mouth, encouraging salivation, which lubricates the mouth and makes it softer, whereas fixed ring bits are more precise in action than the loose rings.

Copper and steel/iron mixtures (sweet'n'sour bits) encourage salivation by varying "tastes" of the metal, as in the cherry roller, which also provides play in the rings. Breaking bits have smooth "keys" to encourage play, which is fine when introducing a youngster to a bit, but not favourable for riding in, as it can distract the horse from the job in hand. Nylon and rubber/vulcanite bits can be too dry for some horses, causing sore mouths from rubbing. Vulcanite is hardened rubber.

SNAFFLES

Snaffles vary in severity and if often depends on the material it is made of. Snaffles are more often "broken" in the middle, however not always.
The "MULLEN MOUTH" is unjointed and slightly curved, putting pressure on the tongue and lips.
The "DOUBLE JOINTED" puts less pressure on the bars than the single jointed snaffle.
"SINGLE JOINTED SNAFFLES" which there are many to choose from (full cheek, eggbut, Dr. Bristol & fulmer just to name a few) give guidance by not allowing the bit to be "pulled" through the mouth.
"GAG" bits are technically snaffles with leverage action on the poll.

CURB

Common "CURB" bits include the pelham, kimblewick, liverpool, universal reversible, gags and the weymouth bit of the double bridle.

The curb group acts by applying pressure to the poll and chin groove as well as points within the mouth, to lower the horse's head and tuck the nose in by leverage. Curb bits if used by heavy-handed or novice riders can cause extreme pain to a horse by "squeezing" the head. Also a hard yank on a curb bit with a curb chain attached has been known to break the horse's mandible (jaw bone).

Curb bits (except for gags) are used with a curb chain in order to prevent the bit from moving beyond a 45degree angle , which would put an unacceptable amount of pressure on the poll, by acting as a stop using the chin groove. This is why correct fitting and measurement of the curb chain is vital when fitting a curb bit.

BIT FITTING

Poorly fitted bits can cause a multitude of problems too wide and they will bang and bruise the bars, too narrow and they will pinch the lips. If a horse is dry mouthed they can rub their mouths raw on rubber, vulcanite or nylon, and may be more suited to steel and copper/iron combinations. Copper tastes sour to the horse, but is warmer than steel, and iron is sweet, but rusts easily.

A well fitted bit should have 1/4 of an inch clearance on either side of the lips, and slightly wrinkled at the corners of the lips. Some horses play a lot with the bit when it is first put on, holding it higher in the mouth or similar, so wait a few moments until the horse has settled before making any final adjustments. The bit should not bang against the incisor teeth, and not be able to touch the molars when the reins are pulled taut.