Ekman and Friesen ultimately assembled all these combinations—and the rules for reading and interpreting them— into the Facial Action Coding System, or FACS, and wrote them up in a five-hundred-page document. It is a strangely riveting work, full of such details as the possible movements of the lips (elongate, de-elongate, narrow, widen, flatten, protrude, tighten, and stretch); the four different changes of the skin between the eyes and the cheeks (bulges, bags, pouches, and lines); and the critical distinctions between infraorbital furrows and the nasolabial furrow.
