The imagery of the Refuge Tree, also referred to as Refuge Assembly, Refuge Field, Merit Field, Field of Merit or Field of Accumulation (Tibetan: ཚོགས་ཞིང་།, Wylie: tshogs zhing) is a key part of a visualization and foundational meditation practice common to Tantric Buddhism. Based on descriptions in the liturgical texts of various traditions, Refuge Trees are often depicted in thangkas employed as objects of veneration, mnemonic devices and as a precursor to the contents being fully visualized by the Buddhist practitioner during the Refuge Formula or evocation. While the concept of Refuge Trees appears in liturgical texts at least as early as the 16th century, based on known examples Refuge Tree paintings appear to have only become popular from the 18th century making them a late development in the history of Tibetan art. Content Refuge Tree or Refuge Field paintings depict the important objects of "Refuge" for each sect or lineage in the form of a genealogical chart. Each lineage has its own distinctive form of composition but they usually include the "Three Jewels" (Sanskrit: triratna): Buddha, Dharma and Sangha, the "Refuges" common to all major schools of Buddhism. They may also include what is known in the Nyingma and Kagyu sects as the "Three Roots" (Tibetan: tsa sum) which include the numerous important Lamas and Lineage Holders, particular meditational deities (Tibetan: Yidam), the Dakinis (Tibetan: Khadroma) or the Protectors of the Lineage (Sanskrit: Dharmapāla, Tibetan: Chokyong). Many other figures such as Buddhist practitioners, animals, offering goddesses and other Buddhist symbols and imagery are also often included. A Refuge Tree painting may contain over a hundred figures, or only the key representative figures of each main grouping. A key function of a Refuge Tree painting is to depict the transmission lineage of the particular sect or tradition. As David P. Jackson & Janice Jackson stated in Tibetan Thangka Painting, "For the religious practitioner these paintings embodied the whole lineage through which the tradition descended, from its ultimate origin down to the practitioner's own teacher". The Jacksons go on to explain that the practitioner imagines the main central figure of the Refuge Tree (the identity of which depends on the particular tradition) as the essence of one's main teacher or Root Lama. Structures and compositions The structures of the compositions symbolize the interconnectedness of the various figures and groupings. The figures of "Refuge" are depicted within the form of a tree