The number of π-electrons in the molecules of an aromatic compound satisfies which rule?

According to Hückel's Rule of Aromaticity, a planar, cyclic, continuously conjugated ring system shows aromatic stability if its delocalized cloud contains exactly (4n+2)π-electrons, where n is a non-negative integer (n=0,1,2,3,…).
Common examples include benzene (6π-electrons, where n=1) and naphthalene (10π-electrons, where n=2).