When bases pair between nucleotides loops (hairpin or internal) and bases outside the enclosing loop, they form a pseudoknot. This structure often contains coaxial helices. It can be a very stable tertiary interaction.

Reference: Pleij,C.W.A., K.Rietveld, and L.Bosch. 1985. A new principle of RNA folding based on pseudoknotting. Nucl. Acids Res. 13:1717-1731.