mouth shape
The mouth gives information about anger, dominance, aggression, fear, attention, interest, and relaxation.
mouth relaxed, slightly open, tongue slightly visible: content relaxed dog
mouth closed no teeth or tongue visible: sign of attention or interest. dog is trying to determine the meaning of what he is observing, and evaluating the actions he is going to take.
lips curled to expose some teeth, mouth still mostly closed: sign of annoyance or threat "Go away you are bothering me."
lips curled, major teeth showing, nose wrinkled, mouth partly open: only tells you the intentions and feelings of the dog not what is going on to cause it. "If you do something I interpret as a threat I will bite."
lips curled exposing teeth and gums above the front teeth, with visible wrinkles on nose: last warning that a physical attack is not only possible but might be any moment.
Fear, Anger, or Dominance?
Threats can be triggered by any attempt to assert social dominance also by anger, or annoyance, and can be initiated by fear.
with a confident dog that is angry or annoyed the threatening will go away after the individual bothering it goes away. You can tell the level of aggression by the degree of which the teeth and gums are exposed. The nature of the lip curl and the shape of the mouth opening will tell you what triggered this expression.
Dogs showing fear aggression are not less likely to bite, a fearful dog will defend itself with more intensity than a dominate dog.
Head positions:
pointing the muzzle at someone is a threat used by dominate or aggressive dogs.
turning the mouth away is saying that they are not a threat.
Ears:
The ears give very significant signals. they should be read in context with the whole activity of the dog. When used with other signals the dog may give adds greatly to the clarity of the message and permit the addition of certain meanings.
Ears erect or slightly forward: dog is studying environment or a sign of attention
Ears erect, head tilted slightly, open mouth: means "this is really interesting"
ears erect, mouth closed, head tilted, eyes squinted a bit: means "I don't understand that."
Ears pulled back Flat against the head: means "I am frightened and I will defend myself"
Ears held flat, mouth drawn back, no visible teeth, and smooth forehead: submissive gesture meaning the dog likes you and realizes you are the dominate one.
Flattened ears, relaxed open mouth, blinking eyes and high tail: Means I like you and we can have fun together.
Ears pulled slightly back to the sides forming a v shape: basically means "I do not like this"
Eyes:
Direct eye to eye stare: threat, expression of dominance, or announcement of an attack
eyes turned away to avoid direct eye contact: sign of submission or fear
Blinking: breaks the dominance stare and shows submission
Eye shape:
the larger and more rounder the visual eye is the more threatening and angry the dog is.
Smaller, less visible, and narrower eyes are associated with fear, submission and pacification