Six of their names on one stone and the remaining six names on the other stone, according to their birth.
There were two stones on the shoulders of the ephod (a garment worn by the Kohein Gadol), each bearing the names of six Tribes. The Talmud in Sotah (36a-b) discusses the matter at some length and cites a tradition that the names were divided so that there were 25 letters on each stone. The problem is that this necessitates writing the names of Jacob’s sons not in the order of their birth! The Talmud answers by explaining that “according to their birth” doesn’t mean “in the order of their birth.” Rather, it refers to the names that their father gave them at birth, as opposed to the names by which Moshe referred to them later in the Torah. For example, the stones said Reuven and Dan, not Reuveni and HaDani (Reubenite and Danite).