A cusp is a (pointed or rounded) projection on the chewing surface of a tooth. In humans the premolars are known as the bicuspid teeth, owing to the presence of two cusps on their chewing surfaces. Mating area of the teeth increases from the midline towards the posterior of the jaw. Thus the molars have a much bigger grinding and chewing surface than the canine teeth which are pointed and have virtually no grinding area.
The bicuspids lie between the canines and the molars in each jaw and have a surface area which is intermediate between that of canines and molars. They combine the functions of the canines and the molars. That is incising and grinding, respectively. For this this reason they are also called transitional teeth. By natural design food is passed from the incisors to the bicuspids or premolars, and then to the molars along route of increasing ability to chew and grind. There are two bicuspids-first bicuspid and second bicuspid-in each quadrant of an adult dentition making a total of eight bicuspids in the adult mouth.
Premolars or bicuspids have at least two cusps each in their mating surfaces. Cusps in one tooth are different sized. Mandibular bicuspids tend to have the buccal (towards the cheek) cusps that are larger, especially the first bicuspid. The maxillary bicuspids, especially the second one has two cusps on the lingual (towards the tongue) side. This is because the upper teeth lie ahead of the lower teeth in the front of the mouth, but gradually the overlap between the upper and lower teeth increases towards the rear.
Bicuspids (premolars) differ from the molars in terms of shape and size, and priority in eruption (in addition to location). Molars are significantly larger, have four cusps, and in general, erupt later. Bicuspids usually have a single root, except the maxillary first bicuspid which usually has two roots. In contrast with bicuspids the molars all have two roots. The Bicuspid teeth are absent in the primary dentition.