The early history of the Hittite kingdom is known through tablets that may first have been written in the 18th century BC, in Hittite; but most of the tablets survived only as Akkadian copies made in the 14th and 13th centuries BC. These reveal a rivalry within two branches of the royal family up to the Middle Kingdom; a northern branch first based in Zalpuwa and secondarily Hattusa, and a southern branch based in Kussara (still not found) and the former Assyrian colony of Kanesh. These are distinguishable by their names; the northerners retained language isolate Hattian names, and the southerners adopted Indo-European Hittite and Luwian names.