For any system to be aromatic, it must possess the following three necessary characteristics:

(i) Planarity: The ring system must be completely planar.

(ii) Complete delocalisation of the $\pi$ electrons: The $\pi$ electrons in the ring must be completely delocalised.

(iii) Hückel Rule: The system must have a presence of $(4n + 2) \pi$ electrons in the ring, where $n$ is an integer $(n = 0, 1, 2, \ldots)$.

These conditions apply to all ring systems, whether or not they contain a benzene ring.