Nitrogenous bases in DNA

C & T are single-ringed pyrimidines; A & G are double-ringed purines. Note that the main purine ring is a a pyrimidine ring, with the same numbering system, and the secondary rings are identical. The attachment of NH2 , CH3, and C=O groups makes each base unique. Compare the structure of thymine with that of uracil, which is a pyrimidine base  found in RNA. A common mnemoic device to remember the difference is that the "Pyramids were CUT from stone."

Figure 2012 TA Brown, Introduction to Genetics (1st ed.); additional text 2013 by Steven M. Carr