Following the Kālacakra materials, Situ's catalog classifies the remaining tantras of the “Highest Yoga” class by dividing them between “Mother” and “Father” tantras. According to this traditional scheme, Mother Tantras are those that primarily focus on “wisdom” or “insight” (ཤེས་རབ་, shes rab, prajñā), while Father Tantras primarily focus on “methods” (ཐབས་, thabs, upāya).
The texts here in the Mother Tantra section are some of the most important esoteric materials in the entire canon, and include the works associated with the major tantric systems of Cakrasaṃvara and Hevajra. The individual texts are grouped according to their associations with six buddha-families: