Sometimes more than one valence structure is possible and there shows to be no unique assignment. A familiar organic example is in the disposition of single and double C-C bonds in benzene. In the carbonate ion 11) the three structures shown are equivalent by symmetry, and experimentally all three C-O bonds have same length. We describe this situation as resonance between the different structures, and represent it by the double headed arrows shown in 11. The term is misleading as it suggests a fast oscillation between different structures, which certainly does not happen. It is better to think of a wavefunction that is contracted by combining the structures, none of which on their own describe the bonding correctly.
Resonance may also be appropriate with different valence structures that are not equivalent but look equally plausible, as in nitrous oxide (N2O 12).