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The Three Traditions of Bonpo Dzogchen
In general, within the Bon tradition, there exist different lines of transmission for the Dzogchen teachings which are collectively known as A rdzogs snyan gsum. The first two of them represent Terma traditions based on rediscovered treasure texts, whereas the third is an oral tradition (snyan brgyud) based on a continuous transmission through an uninterrupted line of realized masters. These three transmissions of Dzogchen are as follows:
The first cycle here of Dzogchen teachings is called A-khrid (pronounced A-tri), that is, the teachings that guide one (khrid) to the Primordial State (A). The white Tibetan letter A is the symbol of Shunyata and of primordial wisdom. The founder of this tradition was Meuton Gongdzad Ritrod Chenpo, who was frequently just known as Dampa, "the holy man."  He extracted these Dzogchen precepts from the Khro rgyud cycle of texts. Together with the Zhi-ba don gyi skor, these texts formed part of the sPyi-spungs yan-lag gi skor cycle of teachings that belong to the Father Tantras (pha rgyud) originally attributed to Tonpa Shenrab in the guise of Chimed Tsugphud ('Chi-med gtsug-phud). To this collected material, Meuton added his own mind treasure (dgongs gter) and organized the practice of the cycle into eighty meditation sessions extending over several weeks. This was known as the A-khrid thun mtsham brgyad-cu-pa. The instructions were divided into three sections dealing with the view (lta-ba), the meditation (sgom-pa), and the conduct (spyod-pa). Upon a successful completion of the eighty session course, one received the title of Togdan (rtogs-ldan), that is, "one who possesses understanding."
The system was later condensed by his successors. In the thirteenth century Aza Lodo Gyaltsan  reduced the number of sessions to thirty and subsequently in the same century Druchen Gyalwa Yungdrung wrote a practice manual in which the number of sessions in retreat (thun mtsham) was further reduced to fifteen. This popular practice manual is known as the A-khrid thun mtsham bco-lnga-pa.  And in the present century, the great Bonpo master Shardza Rinpoche wrote extensive commentaries on the A-khrid system, together with the associated dark retreat (mun mtshams).  The A-khrid tradition, where the practice is very systematically laid out in a specific number of sessions, in many ways corresponds to the rDzogs-chen sems-sde of the Nyingmapa tradition. 
Here the term rDzogs-chen does not mean Dzogchen in general, but the reference is to a specific transmission of Dzogchen whose root text is the rDzogs-chen yang-rtse'i klong-chen, "the Great Vast Expanse of the Highest Peak which is the Great Perfection," rediscovered by the great Terton Zhodton Ngodrub Dragpa in the year 1080. This discovery was part of a famous cycle of treasure texts hidden behind a statue of Vairochana at the Khumthing temple at Lhodrak. This root text is said to have been composed in the eighth century by the Bonpo master known as Lishu Tagring. 
The third cycle of transmission of the Dzogchen teachings within the Bon tradition is the uninterrupted lineage of the oral transmission from the country of Zhang-zhung (Zhang-zhung snyan-rgyud), which is the subject of the present study. Because this tradition has a continuous lineage extending back to at least the eighth century of our era, and so does not represent Terma texts rediscovered at a later time, it is of particular importance for research into the question of the historical origins of Dzogchen. [Excerpted from Space, Awareness, and Energy: An Introduction to the Bonpo Dzogchen Teachings of the Oral Tradition from Zhang-zhung, by John Myrdhin Reynolds, Snow Lion Publications forthcoming in 2001.]