How did Joseph H. Greenberg classify Niger-Congo languages?

Joseph H. Greenberg revolutionized the study of African languages in the 1940s and 1950s by establishing that the languages previously categorized by Diedrich Westermann as Western Sudanic and the Bantu languages actually formed a single genetic family, which he named Niger-Congo.1

Greenberg’s classification was defined by the following principles and methods:

• Mass Comparison: His primary methodology involved comparing basic vocabulary word lists across a large number of languages to establish cognates, rather than relying on general typological features (such as the presence of noun classes) that might not indicate a shared origin.12
• Detailed Comparison: He insisted that classifications must be substantiated by a detailed comparison of the actual forms and functions of grammatical morphemes to ensure they were not merely coincidental similarities.1
• Genetic Unity: By grouping these languages together, he provided a unified framework that reflected their predominance in the Niger and Congo river basins.1

While his work has been subject to debate, most of his conclusions regarding the Niger-Congo family are generally accepted by scholars today.2