Bhai Gurdas was Guru Arjan's scribe — his hand physically wrote the Adi Granth in 1604 — and he was the nephew of Guru Amar Das and a preacher in his own right. His forty vars and 556 kabitt savaiyye are the first attempt to explain Sikh doctrine rather than reveal it, and the Gurus are said to have called them the key to the Guru Granth Sahib. They are also the earliest source for a great deal of Sikh history, including the list of prominent Sikhs of each Guru, which historians use because nothing earlier survives. His writing is the only text outside the scripture and the approved Dasam Granth banis that the Rehat Maryada permits to be sung from a gurdwara stage. That single permission tells you how tightly the boundary around scripture is drawn.