Cephalic index is the ratio of the maximum width of the head multiplied by 100 divided by its maximum length (i.e., in the horizontal plane, or front to back). The index was widely used by anthropologists in the early twentieth century to categorize human populations, and by Carleton S. Coon in the 1960s. Today it is mainly used to describe individuals' appearances and for estimating the age of fetuses for legal and obstetrical reasons. The index is used to categorize animals, especially dogs and cats.