The basic principle of a snaffle bit is that control over the horse is achieved through direct pressure on the corners of the mouth and on the lower jaw. The general effect of this pressure is to draw the head upwards and inwards.
There is great variation in snaffle bits, which allows them to put more or less pressure on the three points of contact: the lips, bars, and tongue. Bits like the full-cheek and dee ring snaffles are designed to put more pressure against the sides of the lips and hence aid in turning control. Unjointed snaffles, like the mullen mouth, put more pressure on the tongue, while jointed bits transfer more of that pressure to the bars.
We will consider three types of variation in the snaffle bit: the type of joint(s) or lack thereof, the type of cheek piece or ring to which the reins are attached, and the contours of the mouthpiece. These three elements can be put together in almost limitless combinations.