Purine

 purine  pyrimidine
   A         T
   G         C

In RNA, the complement of A is uracil instead of thymine:

 purine  pyrimidine
   A         U
   G         C

These hydrogen bonding modes are for classical Watson-Crick base pairing. Other hydrogen bonding modes are seen in both DNA and RNA, although the additional 3'-hydroxyl group of RNA expands the configurations through which RNA can form hydrogen bonds.