Most curb bits have some degree of port, which is to say an elevated curve in the middle of the mouthpiece. The port makes room for the horse’s tongue to rise up when the bit is engaged, thus putting less pressure over the middle of the tongue and more pressure over the bars. Ports vary in shape by how high they are – a low port puts more pressure over the tongue, but has more clearance for the palate, while a high port gives more room to the tongue, but has a greater danger of interfering with the palate.

Most ports seek a compromise between these two effects, although one alternative is the forward angled port, or Conrad curb. Ports also vary in width, with the main consideration being the width of the tongue groove. On average, 34mm (1 1/2 inches) gives maximum tongue clearance, while still allowing the sides of the mouthpiece to lay comfortably on the bars (which are on either side of the tongue groove). Wide port curbs are made for horses that have particularly wide tongue grooves.