The lips open (close) or protrude (retract) relative to the jaw and maxilla. Point L denotes the position of the lower lips, having coordinates (PL,HL) with respect to point J, which is fixed with respect to the jaw. Use of HL and PL as separate variables allows lip closure with spread or rounding lips. The protrusion and elevation of the upper lip relative to the upper incisors are assumed equal to those of the lower lip relative to the lower incisors. Total lip opening is again given by two components: one a function of the jaw opening Øj, and the other lip opening HL relative to the jaw.