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 cephalic index is used to classify animals into three groups: Brachycephalic (literally 'short-headed'): the length of the cranium is shorter than the width, giving the top and sides of the cranium a round shape, often referred to as 'apple-head'.
Mesaticephalic or mesocephalic ('middle-headed'): the length and width are equal, giving a square shape.
When dealing with animals, especially dogs, the more appropriate and commonly used term is not "mesocephalic", but rather "mesaticephalic", which is a ratio of head to nasal cavity.
Breeds of each group are listed in Cephalic index.