FACS is a tool developed by Paul Ekman and Wallace V. Friesen in the 1970s. It is a comprehensive facial coding system used to analyze and describe facial expressions. FACS consists of a set of anatomical markers on the human face, such as facial muscles and movements, and their corresponding action units (AUs). These AUs are the building blocks of facial expressions and can be combined to form various facial expressions. FACS has been widely used in fields like psychology, anthropology, computer vision, and emotion research. It is particularly useful in studying nonverbal communication, emotions, and analyzing facial behavior in different contexts.