The Chihuahua breed standards of the AKC, FCI, UKC, and Chihuahua Club of Canada all call for a well rounded “apple dome” head, but an “apple head” is singled out as a fault in the American Hairless Terrier, Toy Fox Terrier, and Keeshond, and as a less than desirable head in the Brittany and the Pug.

Though an “apple head” is a term sometimes used for congenital hydrocephalus, in healthy dogs it refers to a convex top skull, a round “apple-like” head with a 90-degree angle where the muzzle joins the forehead. A prominent frontal bone is what gives a an apple skull its shape. 

This juncture, which should essentially form an “L” shape, is the most distinguishable feature of an apple-head Chihuahua. Such dogs have shorter and more prominent jaw lines than deer head Chihuahuas, and because of the shape of their skulls, apple head Chihuahuas tend to have more prominent and expressive eyes. They are the only type of Chi that is competitive in a show ring.

In the American Hairless Terrier, an abrupt stop is a serious fault,  so it follows that an apple head with its “L” shape juncture would preclude a correct stop. Similarly, the Toy Fox Terrier standard calls for the skull to be only slightly rounded with a somewhat sloping, medium stop and again, the more pronounced rounded dome of an apple shaped head would foil a correct stop.

A Pug’s head is large, massive and round, it’s true, but the standard writes that should be there no indentation of the skull. In an apple head, there is an irregular roundness of the top skull in various degrees that curves inward toward its center or indentation of the skull (Encyclopedia of K-9 Terminology).