Post-10th century Tibetan classification schemes4 that were formalized by the 14th century in the structure of the Tibetan Kangyurs typically catalogue the translated texts of the Yoginītantra class, which Tibetans called “Mother tantras,” alongside what they called “Father tantras”5 and “Non-dual tantras,”6 to make up the more inclusive category of “Unexcelled Yoga Tantra” (yoganiruttara / anuttarayoga tantra).7
The Unexcelled Yoga tantras, believed by most Tibetan exegetes to be the ultimate revelation of the Buddha of our eon, therefore occupy the first major bibliographic category in the “tantra collection”8 of most Kangyur collections.
Unexcelled Yoga tantras are in turn followed by the tantra classes of “Yoga,” “Conduct” and finally “Action,” an order thought to represent along a descending gradient the relative soteriological power of the yogic techniques emphasized in each textual class.
The same hierarchical logic is also reflected in the Kangyur’s internal sub-divisions of Highest Yoga tantra itself, where the highest of the high, Non-dual tantras, are followed by Mother tantras and then by Father tantras, once again reflecting in descending order Tibetan conceptions of the relative profundity and power of each sub-class’s methodological emphasis.
In relating the rationale for this tripartite hierarchy in his commentary on the Guhyagarbhatantra, Longchenpa (klong chen rab ’byams pa dri med ’od zer, 1308–1364) echoes the following popular tantric dictum: