They have large round eyes and their heads are well domed with their ears being set high and well apart. Chugs have nicely furrowed brows which is a physical trait they inherit from the Pug and the Chihuahua. Muzzles can be broad and wide or shorter and slimmer depending on which parent they throw to. Jaw lines are distinctly Pug-like with Chugs often having slightly over-shot jaws. Muzzles can be short or they can be longer depending on which of their parent breeds a Chug leans towards as such they can be more brachycephalic than dogs with longer muzzles which means they often have trouble breathing.