Bits - To choose a bit for your horse, first you must learn about the different parts of a bit and what each part does.  Then you will be able to assess each individual horse's needs.  As a horse progresses with his training, his needs in regards to bitting will change, and a basic working knowledge of bit and how they work will help you to be sure that you are always using the best bit for the situation.  First, let's look at the anatomy of a bit.  The cheek is the total length of the bit from the headstall connection to the rein connection.  AQHA rules state that the bit is measured to the center of the bottom ring, where your reins are attached, but not everyone measures that way.  If you are in competition, you should always measure the cheek length yourself and verify with your association how their rules state measurements should be taken.  Often the cheek is incorrectly referred to as the shank.  In actuality, the shank is the portion of the bit below the mouthpiece.  The purchase is the portion of the shank above the mouthpiece.  These two parts of the cheek work together in basic leverage.  The longer the shank portion is, the more leverage is applied to the mouth of the bit.  Shorter purchase will speed up the action of the bit, making it quicker to engage.  So keep in mind that a longer shank makes a bit more severe while a longer purchase makes it milder.

Moving on to the mouth of the bit, there are several different areas inside the horse's mouth that are affected by the bit.  The bars are the gum area between the horse's front and back teeth where the bit actually rests in the mouth.  Correspondingly, the bars of a bit are the portion of the bit that contact there.  Almost all bits apply pressure to the bars in some degree.  Palate pressure is applied to the top of the horse's mouth by the port portion of a bit.  The corners of the mouth are affected most by snaffle bits, but to some degree by others.  Curb pressure is applied to the chin.  Any shank bit used with a curb strap or chain will give you curb pressure.  Another important area that bits affect, but is often overlooked, is the poll.  All shank bits will have some degree of poll pressure.  Leverage created by pull on the shanks will cause the crown of your bridle to tighten, applying poll pressure to the top of the horse's head. 

Different types of mouthpieces will apply more pressure to some areas and less to others.  The two piece snaffle is one of the most common bits.  It works primarily off the bars and the corners of the mouth.  A three piece snaffle, in western circles this is usually a lifesaver or dog-bone, adds tongue pressure and changes the area of the bars that pressure is felt on by the horse.  This usually gives you a little extra collection and helps the horse to engage.  Floating spade bits keep the tongue pressure of a three piece but introduce palate pressure.  Correction bits have a port that gives a horse some tongue relief, or a lessening of pressure, because it gives the tongue some extra room.  The port applies pressure to the palate, in addition to having bar pressure.  The broken mouth is excellent for lateral work.  Solid port bits typically have a lot tongue relief and work off of palate, curb, and bar pressure.  They keep a horse stable and are best used when neck reining, and are not the best suited for lateral and flexion exercises.