Cephalic inex is the ratio of the maximum width of the head to its maximum length (i.e., in the horizontal plane, or front to back), sometimes multiplied by 100 for convenience. It was once widely used to categorize human populations, but is no longer used for that purpose except for describing individuals' appearances, and has no correlation with behavior. It is still sometimes used for estimating the age of fetuses for legal and obstetrical reasons, in which case the differences in skull shapes between different populations are still of interest. Such an index is also used to categorize animals, especially dogs and cats.