Abstract
There i s increasing evidence that multiple shell structures occur often in planetary nebulae and emission nebulae surrounding Oef and Wolf-Rayet Stars, yet the multiple shell structures in the two classes of objects are very different. In planetary nebulae they are defined by an internal structure filled by [OIII] 5007 emission, and an outer shell that is most noticeable in the light of singly-ionized species such as [Oil], [SII] or [Nil]. Planetary nebulae with multiple shell structure are well exemplified by NGC 6853 (The Dumbbell Nebula) and NGC7293 (The Helix Nebula). The most pronounced effect is [OI] condensations plus [NII], [OII], [SII] “shadows” and complementary [OIII] structure.
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