He taught Romance languages and literature at the universities in Tübingen and Munich.

His main interest was the languages and dialects spoken in Southern Italy and he travelled extensively in this region.

He studied Italiot Greek (a language still spoken in a few places in Salento, southern Apulia, and in Bovesia, southern Calabria) and found several indications suggesting that Italiot-Greek is a direct descendant of the language originally spoken by the Greek colonists of Magna Grecia.

He first advanced this theory in his book Griechen und Romanen in Unteritalien (Greeks and Romans in Southern Italy, 1924).

His main work is considered to be his Historical Grammar of the Italian Language and its Dialects (Historische Grammatik der italienischen Sprache und ihrer Mundarten, 1949–1954).