into editions called sungbum. The commentaries and interpretations that are used to shed light on these texts differ according to tradition. The Gelug school for example, use the works of Tsongkhapa, while other schools may use the more recent work of Rimé movement scholars like Jamgon Kongtrul and Jamgon Ju Mipham Gyatso. A corpus of extra-canonical scripture, the treasure texts (terma) literature is acknowledged by Nyingma practitioners, but the bulk of the canon that is not commentary was translated from Indian sources. True to its roots in the Pāla system of North India, however, Tibetan Buddhism carries on a tradition of eclectic accumulation and systematisation of diverse Buddhist elements, and pursues their synthesis. Prominent among these achievements have been the Stages of the Path and mind training literature, both stemming from teachings by the Indian scholar Atiśa. Tantric literature In Tibetan Buddhism, the Buddhist Tantras are divided into four or six categories, with several sub-categories for the highest Tantras. In the Nyingma, the division is into Outer Tantras (Kriyayoga, Charyayoga, Yogatantra); and Inner Tantras (Mahayoga, Anuyoga, Atiyoga/Dzogchen), which correspond to the "Anuttarayoga-tantra". For the Nyingma school, important tantras include the Guhyagarbha Tantra, the Guhyasamaja Tantra, the Kulayarāja Tantra and the 17 Dzogchen Tantras. In the Sarma schools, the division is: Kriya-yoga - These have an emphasis on purification and ritual acts and include texts like the Mañjuśrīmūlakalpa. Charya-yoga - Contain "a balance between external activities and internal practices", mainly referring to the Mahāvairocana Abhisaṃbodhi Tantra. Yoga-tantra, is mainly concerned with internal yogic techniques and includes the Tattvasaṃgraha Tantra. Anuttarayoga-tantra, contains more advanced techniques such as subtle body practices and is subdivided into: Father tantras, which emphasize illusory body and completion stage practices and includes the Guhyasamaja Tantra and Yamantaka Tantra. Mother tantras, which emphasize the development stage and clear light mind and includes the Hevajra Tantra and Cakrasamvara Tantra. Non-dual tantras, which balance the above elements, and mainly refers to the Kalacakra Tantra The root tantras themselves are almost unintelligible without the various Indian and Tibetan commentaries, therefore, they are never studied without the use of the tantric commentarial apparatus. Transmission and realization There is a long history of oral transmission of teachings in Tibetan Buddhism. Oral transmissions by lineage holders traditionally can take place in small groups or mass gatherings of listeners and may last for seconds (in the case of a mantra, for example) or months (as in