Carefully observing the way the mouth looks when relaxed and open will help us see more clearly the way its shape changes when we're happy, angry, or sad. The oval shape of the opening is the key. In most expressions it squares off dramatically; in none (except surprise) is it oval. Note that as mouth drops, lower lip stretches more than upper, as it follows dropping teeth.
Think of the upper lip as divided in thirds. The center third, with dip and peaks, is held in place by teeth. The outer legs "hinge" off the center and angle down to mouth corners. The lower lip appears thicker because the wide upper surface (A), normally covered by the upper lip, is exposed. The top and front of the lip are seen.
There is a sharp break where the inner edges of the two lips meet (B). The lower lip tends to bend right in the middle, forming a V or wishbone shape; the legs curve up to corners and are less angular than the upper lip. When the open mouth is relaxed, only tips of the teeth show and only in the center of the mouth.
This pose suggests singing or yawning. With the jaw opened as much as possible, the cone-like form of the mouth setting is most apparent through stretched skin (C). Creases bracket the mouth on either side. Narrowing of the face is made even stronger by the stretching of the skin from cheekbone to chin (D). Often the outline here can be concave, with a hollowed-out look.
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