DNA and RNA structures are defined by Watson-Crick rules of hybridization. However, a variety of DNA and RNA structures have been defined, which do not rely on simple A-T and C-T/U binding. Since these structures disobey the Watson-Crick canon, they are described as non-canonical. Non-canonical DNA and RNA segments are formed as a result of secondary structures. These include G-quadruplexes, triplex forming oligos, hairpins and cruciforms, and i-Motif structures.