Cephalic index in cats and dogs Wikipedia
The cephalic index of a vertebrate is the ratio between the width (side to side) and length (front to back) of its cranium (skull). This ratio does not concern the muzzle or face, and thus is distinct from the craniofacial ratio, which compares the size of the cranium to the length of the muzzle. The two measures are often confused in descriptions of dog breeds.
The cephalic index is used to classify animals into three groups:
Breeds of each group are listed in Cephalic index#Use in animal breeding (Modern).