The nitrogenous bases have a ring of nitrogen and carbon atoms with various functional groups attached. There are two types of nitrogenous bases. They differ in their structure: thymine and cytosine have a single-ring structure, while adenine and guanine have double-ring structures. When James Watson (1928–) and Francis Crick (1916–2004) were imagining how the bases would join together, they knew that the pairing had to be such that there was always a uniform diameter for the molecule. It therefore became apparent that a double-ring base must always be paired with a single-ring base on the opposite strand.