Base pair classification of Leontis and Westhof. The Leontis-Westhof classification system annotates basepairs according to the interacting edge used by each base (Watson-Crick, Hoogsteen, or Sugar), and glycosidic bond orientation (cis, trans). For six of twelve classes, the correct Leontis-Westhof notation depends upon the order (i,j) of the identified nucleotides. Leontis NB and Westhof E (2001) Geometric nomenclature and classification of RNA base pairs. RNA 7:499-512. https://doi.org/10.1017/s1355838201002515 Replacement for category item _ndb_struct_na_base_pair.hbond_type_12. The older item encodes the 12 classes by a numerical index, without identifying the interacting edges used by each base in nucleotides i,j.