Friction introduces a phase shift between forced oscillations of the pivot and harmonics of the steady-state n-periodic motion of the pendulum. By virtue of this phase shift the pendulum is supplied with energy needed to compensate for frictional losses. With friction, the direct and backward spatial paths of the pendulum do not coincide, and the symmetry of the phase trajectory with respect to the ordinate axis is destroyed. This is clearly seen from a comparison of Figures 4 or 5 for subharmonic resonances in the presence of weak friction with Figure 2, which refers to an idealized case in which friction is absent.