There are four Vedas - known as the Rig Veda, Yajur Veda, Sama Veda and Atharva Veda. The Rig Veda is the primary one and it is the foundation of all Indian thought, philosophy and religious consciousness. It is in poetic form; there are about 10,000 mantras. The Yajur Veda is partly in poetry and partly in prose. The Sama Veda is comprised of musically set verses, mostly from the Rig Veda, and they are sung in a melodious tune. The Atharva Veda is filled with a variety of subjects such as technology, art, and other scientific thoughts with which we are familiar in this world. Religiously, spiritually and philosophically, only three Vedas are important - Rig Veda, Yajur Veda and Sama Veda - and, therefore, they are called the Trayi in Sanskrit. Trayi means the threefold knowledge: Rig Veda, Yajur Veda and Sama Veda.

The Bhat-Tray (literally "the great triad (of compositions)") refers to three early Sanskrit encyclopedias of medicine (Skt yurveda). These are contrasted with the Laghu-Tray or the "lesser triad", a secondary set of later authoritative compositions.

This classification was first devised probably at some time in the 19th century, although its earliest use has not yet (2008) been identified. The classification is not known to authors before the 18th century. It is part of an effort to create a formal canon for ayurvedic literature.

There are older medical encyclopedias than are not included in the Bhat-Tray, for example the Bhea-sahit.