One of the most important attempts to describe facial movements was Facial Action Coding System (FACS). Originally developed by Carl-Herman Hjortsjö in the 1960s and updated by Ekman and Friesen in 1978, FACS defines 46 basic facial Action Units (AUs). A major group of these Action Units represent primitive movements of facial muscles in actions such as raising brows, winking, and talking. Eight AU's are for rigid three-dimensional head movements, (i.e. turning and tilting left and right and going up, down, forward and backward). FACS has been successfully used for describing desired movements of synthetic faces and also in tracking facial activities.