The relation between Dongolawi and Nobiin remains a matter of debate within Nubian Studies. Ethnologue's classification is based on glotto-chronological research of Thelwall (1982) and Bechhaus-Gerst (1996), which considers Nobiin the earliest branching from Proto-Nubian. They attribute the current syntactical and phonological proximity between Nobiin and Dongolawi to extensive language contact. Arguing that there is no archeological evidence for a separate migration to the Nile of Dongolawi speakers, Rilly (2010) provides evidence that the difference in vocabulary between Nobiin and Dongolawi is mainly due to a pre-Nubian substrate underneath Nobiin, which he relates to the Meroitic. Approaching the inherited proto-Nubian vocabulary in all Nubian languages systematically through a comparative linguistic approach, Rilly arrives at the following classification: