This paper presents a mathematical framework for generating stable and smooth gait trajectories for bipedal robots. Traditional approaches using the Linear Inverted Pendulum Model (LIPM) often neglect the double-support phase, resulting in discontinuities in acceleration and impulsive forces during foot transitions. To address this, we propose a hybrid planning method that integrates the LIPM for the Single Support Phase (SSP) with a Linear Pendulum Model (LPM) for the Double Support Phase (DSP). This paper details the analytical formulation of both models, the geometric and dynamic boundary conditions required to ensure continuity, and the periodicity constraints necessary for stable cyclic locomotion. The resulting framework provides a computationally efficient method for generating Center of Mass (CoM) trajectories with continuous acceleration profiles.