Kay (title) explained

Kay (meaning "king") was a ruling title used in Iranian mythology by the Kayanians of the Avesta, later to be adopted by the Kushano-Sasanians, followed by the Sasanian monarchs of Iran. Some legendary rulers who held the title include Kay Kavus, Kay Khosrow, and Kay Lohrasp.

Sources

• Book: Rezakhani, Khodadad . ReOrienting the Sasanians: East Iran in Late Antiquity . 2017 . Edinburgh University Press . East Iran in Late Antiquity. 1–256 . 9781474400305 . 10.3366/j.ctt1g04zr8 .
• Book: Sauer, Eberhard. Sasanian Persia: Between Rome and the Steppes of Eurasia. London and New York. Edinburgh University Press. 2017. 9781474401029. 1–336.
• Book: Schindel . Nikolaus . Potts. Daniel T.. The Oxford Handbook of Ancient Iran . 2013 . Oxford University Press . 978-0199733309 . Sasanian Coinage.