Orkhon inscriptions (the Kul Tigin and Bilge Khagan steles)

The Orkhon inscriptions are the Old Turkic royal steles raised for Kul Tigin and Bilge Khagan in the 730s, carved in the earliest known Turkic script and standing as the earliest surviving documentary attestation of Tengri, the sky god, in the historical record. As a primary epigraphic source, contemporary with the events it commemorates, it is the correct ground for dating that attestation and for what the inscription itself says about Tengri and the khagans' relationship to him. Its limit is that it is a royal, commemorative text carved for a ruling dynasty's own self-presentation, not a description of ordinary Turkic religious practice, so a claim about how Tengri was worshipped outside the court needs a source beyond this monument.