cases following their herds; and those who were yogis, often living in caves.Lamas are generally skilled and experienced tantric practitioners and ritual specialists in a specific initiation lineage and may be laypersons or monastics. They act not just as teachers, but as spiritual guides and guardians of the lineage teachings that they have received through a long and intimate process of apprenticeship with their Lamas. Tibetan Buddhism also includes a number of lay clergy and lay tantric specialists, such as Ngagpas (Skt. mantrī), Gomchens, Serkyims, and Chödpas (practitioners of Chöd). According to Samuel, in the more remote parts of the Himalayas, communities were often led by lay religious specialists. Thus, while the large monastic institutions were present in the regions of the Tibetan plateau which were more centralized politically, in other regions they were absent and instead smaller gompas and more lay oriented communities prevailed. Samuel outlines four main types of religious communities in Tibet: Small communities of lay practitioners attached to a temple and a lama. Lay practitioners might stay in the gompa for periodic retreats. Small communities of celibate monastics attached to a temple and a lama, often part of a village. Medium to large communities of celibate monastics. These could maintain several hundred monks and might have extensive land holdings, be financially independent, and sometimes also act as trading centers. Large teaching monasteries with thousands of monks, such as the big Gelug establishments of Sera (with over 6000 monks in the first half of the 20th century) and Drepung (over 7000). In some cases a lama is the leader of a spiritual community. Some lamas gain their title through being part of particular family which maintains a lineage of hereditary lamas (and are thus often laypersons). One example is the Sakya family of Kon, who founded the Sakya school and another is the hereditary lamas of Mindrolling monastery. In other cases, lamas may be seen as "Tülkus" ("incarnations"). Tülkus are figures which are recognized as reincarnations of a particular bodhisattva or a previous religious figure. They are often recognized from a young age through the use of divination and the use of the possessions of the deceased lama, and therefore are able to receive extensive training. They are sometimes groomed to become leaders of monastic institutions. Examples include the Dalai Lamas and the Karmapas, each of which are seen as key leaders in their respective traditions. The