The nitrogenous bases of DNA include the purines adenine (A) and guanine (G), and the pyrimidines thymine (T) and cytosine (C). Chargaff's rule states that there is always a 1:1 ratio of purines to pyrimidines in DNA. More specifically, the #A = #T and the #G = #C.
This is reflected in the molecular structure of the Watson-Crick model of DNA, where the two strands of DNA are held together by hydrogen bonding between nitrogenous bases. Their model shows how A always bonds with T and G always bonds with C.