Three outer classes of tantra
The three outer or lower classes of tantra (Tib. རྒྱུད་སྡེ་འོག་མ་སུམ, ཕྱིའི་རྒྱུད་, gyüdé ok ma sum, chi gyü, Wyl. rgyud sde 'og ma sum, phyi'i rgyud) are classes of tantra which are common to both the Nyingma and Sarma schools of Tibetan Buddhism. They are also called the three tantras of vedic asceticism. They are:
- Tenzin Gyatso, the Fourteenth Dalai Lama, The World of Tibetan Buddhism (Boston: Wisdom Publications, 1995), '18. Tantric Practice: The First Three Classes of Tantra'.
- Tulku Thondup, The Practice of Dzogchen (Ithaca: Snow Lion, 1989), pages 15-22.
- Opening the Wisdom Door of the Outer Tantras: Refining Awareness Through Ascetic Ritual and Purification Practice, Khenchen Palden Sherab Rinpoche & Khenpo Tsewang Dongyal Rinpoche. Dharma Samudra (2008).