Abstract: The opening and closing motions of a push-out casement window are controlled by the linkage between casement and frame. The kinematics of the casement is a typical practical case in the lecture of theoretical mechanics. In this article, a simplified model and a formulation of the problem for this kinematic process are given. Using the cartesian-coordinate method and pole method, the equations of motion, trajectory, velocity, and acceleration of the casement edge are established and analyzed subsequently. In addition, the approach to open the window with the smallest force is discussed.