Dental Arrangement and the Dental Formula

Humans, like other mammals, are heterodont — they possess more than one distinct type of tooth within the jaw, each shaped for a different mechanical task in reducing food to a size and texture suitable for swallowing and digestion — and diphyodont, meaning that two successive sets of teeth are produced over a lifetime rather than one set that lasts throughout, or a continuously replaced series as in some other vertebrates.

Four types of teeth are found in the human dentition. The incisors, at the very front of each jaw, are chisel-shaped teeth with a single sharp cutting edge, used to bite off and cut pieces of food. Immediately behind the incisors, at the corner of the jaw, lies a single pointed, conical canine tooth on each side, adapted for piercing and tearing tougher food (a much-reduced structure in humans compared with carnivores, but still present and functionally important). Behind the canines lie the premolars, teeth with a broader crushing surface bearing two cusps (bicuspid), followed further back by the molars, the largest teeth in the jaw, with a broad, multi-cusped surface used for grinding and crushing food into a fine, easily swallowed pulp. Each of these four tooth types is present, in a fixed number, in each half (left or right) of the upper and lower jaw, and this arrangement is conventionally summarised using a shorthand notation called the dental formula, which states, for one half of one jaw (since the arrangement is bilaterally symmetrical), the number of incisors, canines, premolars and molars in that order.

Humans, like most other mammals, produce two successive sets of teeth over a lifetime. The first set, the deciduous or "milk" teeth, begins to erupt at around six months of age and is complete, all twenty teeth present, by about two to three years of age; it comprises, in each half-jaw, two incisors, one canine and two molars, but no premolars at all, giving the deciduous dental formula 2102/2102 and a total of twenty teeth in the whole mouth. As the jaw grows through childhood, these smaller milk teeth are progressively shed, beginning at about six years of age, and are replaced one by one by the permanent (adult) dentition, which additionally includes two premolars per half-jaw (a tooth type entirely absent from the milk set) and a third molar at the very back of each quadrant that has no deciduous predecessor at all. The full permanent dental formula is therefore 2123/2123 — two incisors, one canine, two premolars and three molars per half-jaw — giving a total of thirty-two teeth when the set is complete. The third molars, popularly called "wisdom teeth," are usually the last to erupt, typically between the ages of seventeen and twenty-five, and in a substantial proportion of people never erupt fully or are absent altogether. …