Back-side gear tooth contact happens when the anti-backlash (or scissor) gears are applied or tooth wedging occurs. An accurate description of the back-side gear tooth mesh stiffness is important to any study on gear dynamics that involves tooth wedging or anti-backlash mechanism. This work studies the time-varying back-side mesh stiffness and its correlation with backlash by analyzing the relationship between the drive-side and back-side mesh stiffnesses. Results of this work yield the general form of the back-side mesh stiffness or gear tooth variation function for an arbitrary gear pair. The resultant analytical formulae are confirmed by the simulation results from Calyx that precisely tracks gear tooth contact without any predefined relations.