Uto-Aztecan languages
He specialized in the study of Athabascan languages, Chinookan languages, and Uto-Aztecan languages, producing important grammatical descriptions of Takelma, Wishram, Southern Paiute. Later in his career he also worked with Yiddish, Hebrew, and Chinese, as well as Germanic languages, and he also was invested in the development of an International Auxiliary Language.