  Since various structural components of planetary nebulae manifest themselves
differently, a combination of optical, infrared, submm, and radio techniques is
needed to derive a complete picture of planetary nebulae. The effects of
projection can also make the derivation of the true 3-D structure difficult.
Using a number of examples, we show that bipolar and multipolar nebulae are
much more common than usually inferred from morphological classifications of
apparent structures of planetary nebulae.
  We put forward a new hypothesis that the bipolar and multipolar lobes of PN
are not regions of high-density ejected matter, but the result of ionization
and illumination. The visible bright regions are in fact volume of low
densities (cleared by high-velocity outflows) where the UV photons are being
channelled through. We suggest that multipolar nebulae with similar lobe sizes
are not caused by simultaneous ejection of matter in several directions, but by
leakage of UV photons into those directions.
