RNA secondary structure:

Most  RNA  molecules  are  single-stranded  but  an  RNA  molecule  should  hold regions  that  can  form  complementary   base  pairing  where  the  RNA  strand loops  back  on  itself in the above figure. If so, the RNA will have some double-stranded regions.  The Ribosomal RNAs (rRNAs) and transfer RNAs (tRNAs) exhibit   substantial   secondary   structure,   as  do  some messenger RNAs (mRNAs).

figure: An example of self-complementarity in RNA, forming an internal double-stranded region; hydrogen bonding between bases is shown by the symbol.