That is, unlike the carbon-only ring like in benzene and napthalene, there are other elements in the backbone, such as in pyrimidine and purine.
... average family size is 3.04. In the city the population is spread out with 27.2% under the age of 18, 8.7% from 18 to 24, 29.5% from 25 to 44, 21.0% from 45 to 64, and ...