Dog Head Terminolgy
The head is the hallmark of the pure breed dog. Without a 'typical' or correct head, a breed is often not uniquely distinct. This section deals with some of the terms pertaining to the head of a dog. It is our hope that an understanding of these terms will assist in appreciating what the breed experts call a typical or beautiful head for its breed.
Stop
The Soft Coated Wheaten Terrier on the right shows what is commonly referred to as the 'stop' which is the indentation between the eyes as is also shown on the Cocker Spaniel on the left. 
Occiput
This is the bone at the back of the skull. In some breeds like the English Setter pictured and the Bloodhound it is well defined while in other breeds it is barely perceptible.
Parallel Head Planes
When a head is described in a Breed Standard as having parallel head planes, this refers to the two planes diagrammatically illustrated here by two separate lines. The first plane lies on a flat skull and extends from the stop to the occiput. The second plane lies on the bridge of the nose and extends from the stop to the end of the nose.
Some Breed Standards specifically ask a flat skull with parallel head planes. Examples of this are the Soft Coated Wheaten Terrier (pictured), the Collie (Rough), the Collie (Smooth) and the Dobermann.
Other Breed Standards call for a flat skull without any mention of head planes. It is wrong to assume these breeds also have parallel head planes. Changes in the relative inclination of the two head planes alters the basic head structure and usually the expression as well.
Foreface or Muzzle
The foreface or muzzle is the whole of the upper area from the eyes to the nose including the lips.
The sides of the muzzle are often referred to as flews in Breed Standards and general dog jargon. Some Breed Standards refer to lips as muzzle while others refer to the flews as muzzle. So, confusion could arise as to what are lips, what are flews and what exactly is the muzzle.
Technically the lower lips blend into the flesh of the chin while upper lips blend into the flesh that covers the top teeth. In both cases, the lips are that area closest to the mouth cavity devoid of hair [Ref 1.]
Haw
When the lower eyelid is too loose, it droops or sags so the eye rim does not hug the eye. The red pouch which then becomes clearly visible is called haw. This is often seen in a breed with excessive head skin like the Basset Hound (pictured) or a Bloodhound.
Scowl
The Chow Chow's expression is referred to as a 'scowl'. The dictionary describes a scowl as wrinkling or contracting the brow in an expression of anger or disapproval. In the Chow Chow, the slightly tilted, small rounded ear gives the dog this scowling expression.
References and Further Reading
[Ref 1] Harold R Spira “Canine Terminology” Harper & Row Sydney 1982 Page 88