What is port height in a western curb bit and how does it affect the horse?

The port is the upward arch in the center of a curb bit's mouthpiece, and its height significantly affects how the bit acts in the horse's mouth — making port height one of the most important variables in western bit selection. A low port — sometimes called a mullen mouth when the arch is very slight — applies pressure broadly across the tongue when the bit rotates. The tongue absorbs most of the pressure, and the pressure is relatively diffuse. Horses with sensitive mouths, horses new to curb bits, and horses that are tongue-sensitive often go better in a low port because the pressure is predictable and spread across a larger surface area. A medium port allows the tongue to escape the direct downward pressure of the mouthpiece when the bit rotates, directing more pressure to the bars of the mouth — the gum tissue between the incisors and molars where the mouthpiece rests. This can be either more or less comfortable depending on the individual horse's bar sensitivity relative to its tongue sensitivity. A high port acts on the palate — the roof of the mouth — when the bit rotates. This is the most complex action and requires the most finished and lightest horse, because a horse that braces against high port pressure has very few places to go with that discomfort and may rear, gape, or become defensive. High port bits, including the spade bits of the vaquero tradition, are designed for horses whose training is so complete that the bit barely needs to be used at all. Anderson and Parelli both teach selecting port height based on the individual horse's mouth sensitivity and training level rather than on tradition, fashion, or what the previous owner used. Putting a horse in a port that is too high for its training level creates exactly the problems — bracing, head tossing, behind the bit — that bit changes are meant to solve.