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

Carleton S. Coon

Carleton Stevens Coon, was an American physical anthropologist, Professor of Anthropology at the University of Pennsylvania, lecturer and professor at Harvard, and president of the American Association of Physical Anthropologists.-Biography:Carleton Coon was born in Wakefield, Massachusetts to a...

 in the 1960s. Today it is mainly used to describe individuals' appearances and for estimating the age of fetus

Fetus

A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...

es for legal and obstetrical reasons.

The index is also used to categorize animals, especially dog

Dog

The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...

s and cat

Cat

The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...

s.
The cephalic index was defined by Swedish

Sweden

Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

 professor of anatomy

Anatomy

Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...

 Anders Retzius

Anders Retzius

Anders Retzius , was a Swedish professor of anatomy and a supervisor at the Karolinska Institute in Stockholm....

 (1796–1860) and first used in physical anthropology

Physical anthropology

Biological anthropology is that branch of anthropology that studies the physical development of the human species. It plays an important part in paleoanthropology and in forensic anthropology...

 to classify ancient human remains found in Europe.