Comparison of nebular-phase spectra for model r0e2 but adopting a volume filling factor f_vol of 1 (smooth ejecta), 0.1, 0.03, and 0.01. As the ejecta clumping is enhanced, the spectra become redder and exhibit lines from ions with a lower ionization state (see e.g., the shift from O II 7324.3 Å to Ca II 7307.6 Å and from Fe II 6247.6 Å to O I 6316.0 Å; only the strongest transitions are labeled).