Three different expansion velocities of a PN: V_HWHM of the spatially integrated [O iii] 5007 Å line profile, the post-shock velocity, V_post, behind the outer shock at R_out, and the shock velocity, Ṙ_out, itself. Shown is the complete time evolution of these velocities for three sequences with the 0.625 M_⊙ central-star model and different metallicities (see figure legend). The filled circles mark the snapshots shown in Fig. 24, which correspond also to the moment of maximum [O iii] emission. The relevant model parameters are listed in Table 3 where also the values of V_HWHM, V_post, and Ṙ_out for the 3   Z_GD sequence, not plotted for clarity, are given. The bumps at age  ≃1000 years are due to the optically thick/thin transition.