The smile is an essential expression for emotion and communication. Smiles form when our lip corners are pulled diagonally by a muscle called “zygomaticus major.” In the Facial Action Coding System (a descriptive tool I use to discuss facial movements), the diagonal action for the smile is referred to as “lip corner puller,” or AU12.

NOTE: For the purposes of this post, “smile” will specifically refer to the action lip corner puller/AU12/zygomaticus major pulling the lip corners.

Open & Closed-Mouth Smiles

closed-mouth smile:

• lips remain closed/together

open-mouth smile:

• lips are parted
• occurs when zygomaticus major’s pull is strong enough to separate the lips
• separation may also be caused by a series of other lip elevator muscles combining in unique ways and at varying degrees (levator labii superioris alaeque nasi, zygomaticus minor, levator labii superioris) to lift the top lip

open-mouth, open jaw smile:

• both lips and jaw are parted
• occurs when the jaw opens
• parted lips may be due to – opening of the jaw, strength of zygomaticus major, induction of additional facial actions listed above

When artists develop a set of expression shapes, they often need to create different states for open mouth shapes, like the smile – i.e. One cannot create a single smile shape and have it exist in both open and closed form without the addition of other shapes.

In the process of putting together shapes to achieve an open-mouth smile, it is commonplace for artists to add actions such as upper lip raiser and lower lip depressor to a closed-lip smile. This method seems intuitive; based on the names of each action, one would assume upper lip raiser is the obvious choice for getting the top lip to separate from the bottom lip. However: Using upper lip raiser to separate the top lip from the bottom lip is bad form.

Why You Should Change What You’re Doing

When the zygomaticus major action is strong,