The internet is severely lacking in accurate references for the lip tightener shape – AU23. If you don’t own a copy of the FACS Manual, chances are – you have been living a lie.

But why?

Lip tightener is notoriously confusing for a number of reasons.

1. Lip tightener is difficult to perform in isolation. People add all kinds of extraneous facial actions when attempting to perform AU23.
   
   
2. Lip tightener is difficult to describe. If you direct someone to perform lip tightener, it’s not easy to describe what you need. Other action units can be illustrated in a number of ways, but you can only get so creative with lip tightener. 
   
   
3. The inventors of FACS didn’t cover everything. The FACS Manual is a wonderful resource, but as you get deeper into facial anatomy, you will start to identify gaps in documented action units. The muscle behind lip tightener, orbicularis oris, is able to tighten the lips in more ways than one (i.e. in ways other than lip tightener). In the Investigator’s Guide (a supplement to the FACS Manual), the original authors admit, “FACS probably does not include all of the visible, reliably distinguishable actions in the lower part of the face.” One of the actions they missed is an action I call “lip cincher,” or “vertical lip tightener.”  Lip cincher tightens the lips on a vertical plane. Traditional lip tightener tightens the lips on a horizontal plane. People frequently mistake lip cincher as the traditional FACS lip tightener.
   
   examples of vertical lip tightener/lip cincher:
   
   
   
   
4. People assume any form of lip tightening is lip tightener. Another issue with novice to intermediate FACS learners is: people take names too literally. If the lips compress from things like lip presser, tighten from movements like lip cincher, many incorrectly assume “tightening is tightening.” Not in FACS, and not in facial anatomy.

Though AU23 appears frequently in speech and is a common component of tensed and emotional expressions, most sources fail to supply proper lip tightener images or videos. Even commonly used references from sources such as Carnegie Mellon, iMotions, and Noldus FaceReader have dropped the ball:

• Carnegie Mellon

• iMotions

• Noldus FaceReader

Rather than displaying a true FACS-based lip tightener, the above resources show blends of other facial actions: 

Learn more about lip presser vs. lip tightener here.