Cyclic polyenes C_{n}H_{n} generally have nonplanar geometry with no conjugation.
Planar conjugated cyclic polyenes are called [*n*]-annulenes.
To be planar they must obey some rules, namely: 1) all angles must be 120°,
2) there must be no severe steric interaction.
This sets the minimum length to 18 (with exceptional length 6).
Annulenes obeying *n*=4*k*+2 rule may have aromaticity (completely delocalized π-orbitals).
For aromatic annulenes there is a critical *n*=14-30 above which there is bond length alteration.
Some polyenes allow Moebius cycling, see e.g. C18H18.

Some particular cases:

- [4] cyclobutadiene [pdb] has planar rectangular geometry and is unstable.
- [5] cyclopentadienyl (anion) [xyz] is aromatic.
- [6] benzene is exceptional.
- [8] cyclooctatetraene [pdb] has nonplanar geometry as expected (its dianion is aromatic).
- [10] cyclodecapentaene has highly strained nonplanar structure of D2 symmetry.
- [14] cyclotetradecaheptaene has unstable planar structure of D2h symmetry.
- [18] cyclooctadecanonaene [xyz] is aromatic with presumably no bond length alteration.