Since Hindu tradition is a complex of many denominations and sub-traditions, there is no central institutional structure that functions as a central teaching authority. Many of the individual denominations and sects maintain systems of checks on the integrity of the tradition in the form of teaching lineages. Instruction has historically been based on oral transmission of the basic sources, along with extensive commentary, both oral and written, on those sources. Perhaps the closest thing to centralized authority is the living master to whom disciples give their uncontested allegiance. That living master embodies both the authority of an unbroken succession of teachers and his or her personal authority as one spiritually connected to the ancient wellsprings of wisdom. Even the Vedas and the associated primary sacred texts grouped under the heading of shruti (“heard”) do not claim the universal adherence of all Hindus.