Sikh teaching holds that the ten Gurus, from Nanak (1469-1539) to Gobind Singh (1666-1708), were successive bearers of a single spiritual light and authority. The tradition's formula is that the light was the same and the way the same, and only the body changed; the Guru Granth Sahib itself records successors being addressed as Nanak, which is the doctrine embedded in the text rather than asserted about it. Succession was by designation rather than by descent, though the later Gurus were of one family, and was marked by the outgoing Guru placing a coconut and coins before the successor and bowing to him.
The decisive development came at the end of the line. Before his death in 1708 Guru Gobind Singh is held to have ended personal guruship and vested it in two: the scripture, thereafter Guru Granth Sahib, and the community of the initiated, the Guru Khalsa Panth. This is why the Sikh scripture is treated as a living Guru rather than as a book about Gurus. It is installed on a throne under a canopy, attended with a whisk, put to rest at night and brought out in the morning, and consulted for the daily hukamnama; and it is why the Sikh Rahit Maryada, the community's code of conduct, forbids bowing to any living person as Guru.
The doctrine has continuing practical force. It grounds the rejection of claimants to living guruship, and disputes with movements whose leaders are accorded guru-like standing, the Nirankaris, the Namdharis, and more recent groups, have turned on it directly, at times violently. It also underlies the authority of the Panth acting collectively, through the Akal Takht and through the gurmata, a resolution taken in the presence of the Guru Granth Sahib.
Historians describe the development of the doctrine over the seventeenth and eighteenth centuries and note that the sources attesting the details of the 1708 succession are of varying date, an ordinary historical observation, and one the tradition's account does not stand or fall by. What is not in dispute is that from the eighteenth century the Sikh community has been organised around its scripture as Guru, an arrangement close to unique among the world's religions in the standing it accords a text.