Languages of India | Indo-European languages | Celtic languages | Slavic languages | Algonquian languages | Turkic languages | Bantu languages | Romance languages | Berber languages | Australian Aboriginal languages | Germanic languages | Goidelic languages | Arawakan languages | North Germanic languages | Indigenous languages of the Americas | Austroasiatic languages | Indo-Aryan languages | Polynesian languages | Northwest Caucasian languages | Munda languages | Mongolic languages | French-based creole languages | Songhay languages | Semitic languages | Dené–Yeniseian languages | Common European Framework of Reference for Languages | Visayan languages | Tupian languages | Tibeto-Burman languages | Pama–Nyungan languages |
Besides research on Native American languages and field work on the Modern American English dialects, he is the father of the Indo-Hittite hypothesis, first formulated in 1926, based on his seminal work establishing the Indo-European character of Hittite (and the related Anatolian languages), with Hittite exhibiting more archaic traits than the normally reconstructed forms for Proto-Indo-European.