Twenty milk teeth give way to a permanent set of thirty-two
Human teeth cut and crush food ahead of digestion, so they count as digestive organs. Like most mammals we are diphyodont: a deciduous set that normally reaches twenty teeth, then a permanent set of thirty-two made up of incisors, canines, premolars and molars.
The four kinds divide the work. Incisors slice, canines tear, and premolars and molars grind. Every tooth sits in either the upper jaw, the maxilla, or the lower jaw, the mandible, with its roots hidden under the gums, and each is built from several tissues of different hardness. Above the neck of the tooth, where enamel meets cementum, lies the crown: a shell of enamel over dentin surrounding the pulp chamber. Below it is the root, coated in cementum and mostly dentin too, usually threaded with pulp canals.
Root counts follow a pattern. Canines and most premolars have one, but the first premolars of the upper jaw and the lower molars usually have two, and upper molars three. Anything beyond that is called a supernumerary root. The same word covers extra teeth: rare fourth or fifth molars count as hyperdontia, while growing too few teeth is hypodontia.
Baby teeth usually start to break through at about six months, sometimes to the infant's distress, although a few children are born with teeth already showing. The primary set has ten teeth in each jaw: central and lateral incisors, canines, and first and second molars, all eventually replaced. The adult set has sixteen per jaw and adds premolars and third molars. Those third molars, the wisdom teeth, typically emerge between 17 and 25, may never erupt or even form, and often need removing when they do appear.
Dentists need a shared vocabulary, which is why dental anatomy doubles as a naming science covering tooth development, appearance and classification, though not how teeth meet in the bite. Three notation schemes dominate. The FDI World Dental Federation system, standardised as ISO 3950, is used worldwide; the Universal Numbering System only in the United States; and the older Palmer notation survives among some practitioners in the United Kingdom. In the Universal scheme, adult teeth are numbered in a back-and-forth sequence running from 1 to 32 around both jaws.
Source: Human tooth
