Single-stranded RNA molecules form hydrogen bonds between the Watson-Crick (WC) edges of the complementary pairs of bases, namely the canonical WC pairs A-U and G-C, and the wobble pair G-U. The set of base-pairs of an RNA molecule is called the secondary structure of the RNA. RNA molecules fold into tertiary structures via complicated interactions inside of the molecules, but the secondary structures are often used as abstract forms of the RNA structure. It is known that the secondary structures are often well conserved between RNAs of the same functional family.