Odontometrics is the measurement and study of tooth size. It is used in biological anthropology and bioarchaeology to study human phenotypic variation. The rationale for use is similar to that of the study of dentition, the structure and arrangement of teeth. There are a number of features that can be observed in human teeth through the use of odontometrics. Odontometrics is the measurement and study of tooth size. It is used in biological anthropology and bioarchaeology to study human phenotypic variation. The rationale for use is similar to that of the study of dentition, the structure and arrangement of teeth. There are a number of features that can be observed in human teeth through the use of odontometrics. The length and width of teeth are measured from front to back and side to side using calipers. These measurements can be made directly from skeletal material or through dental casts of living humans. The measurements that are reported most commonly are maximum crown length and maximum crown breadth. Linking to the Past by Kenneth L. Feder states that 'Dental development...provides a valuable gauge of the level of physical maturity and age.' The first set of teeth, or the lower central incisors, does not begin to appear until the infant is approximately six-and-a-half months old. The rest of the baby teeth, which are called deciduous teeth, will then appear “fairly consistently across the species”, until the child is about two-years-old, when the second upper molars appear; at this point in development, there are twenty teeth in all.