Hermann Osthoff (1847–1909) and Karl Brugmann (1849–1919) were central figures in the circle of German scholars who rejected a doctrinal approach to the study of linguistics. They came to be known as the Neogrammarian school. At the core of their work was the theory that European languages, together with a subset of languages found in central and southern Asia, have a common origin in a single prehistoric language. They called this ancestor Indo-Germanic (known today as Indo-European) and claimed that its descendants are all related to one another by varying degrees of closeness. This six-volume elaboration of this thesis was published between 1878 and 1910. Volume 4 (1881) is devoted entirely to Osthoff's study of vowel gradation in Indo-European languages. He makes it clear that his recent research on vowels has led him to revise his earlier views.