In the rules of DNA base pairing, cytosine always pairs with guanine, and adenine always pairs with thymine. The complementary shape between the two bases that form a pair allows for them to form hydrogen bonds.
A nucleotide is composed of a sugar, a phosphate and a nitrogenous base. The bases can be either a two-ringed purine, such as adenine and guanine, or a one-ring pyrimidine, such as thymine and cytosine. DNA is able to form its double helix structure due to the complementary base pairing present when two strands run antiparallel. The 20 A width present in the alpha helix does not allow enough space for two purines to pair and conversely is too much space for two pyrimidines. Guanine and cytosine complement to form three hydrogen bonds, while adenine and thymine complement to form two. Due to their structure guanine and thymine are unable to form hydrogen bonds, the same goes for adenine and cytosine. This bonding relationship is often referred to as the Watson-Crick rules, named after the scientists that first made the observation. Due to the rules of base pairing, if one strand of DNA is known it is always possible to read the other complementary strand.