What is an English Bit?
There are literally hundreds of varieties of English Bits available. Each has their own particular uses, and most trainers and professional riders have a few favorites that they turn to again and again.
The mouthpiece on a bit does not rest on the teeth, rather, it sits on the interdental space behind the front incisors and in front of the back molars. This area of the mouth is referred to as the "bars." It is important that a bit fit this area properly - bit size is based on the width of a horses jaw at the interdental space.
Roughly speaking there are two broad categories of bits - direct pressure bits, and leverage bits. Most English bits are direct pressure; pressure on the reins acts directly on the mouthpiece with pressure on the bars, tongue and occasionally the roof of the mouth. Leverage bits, on the other hand, have some form of shank on the cheek piece, which transfers rein pressure to the poll and chin groove, while it also applies pressure to the mouth.
Some English Bits are hybrids. A Pelham, for instance, has a two rein design that allows the rider to use either direct, or leverage pressure with the same bit.