During the 1880s, both Lithuanian journalists urging increased use of Lithuanian and Hungarian scholars trying to establish the true relatives of Hungarian utilized theories drawn from linguistics to make arguments about their nations' characters, despite the different focus and wide separation of their debates. I use these cases to study the interaction between the understandings of language apparent in linguistic texts, which during the mid- to late nineteenth century created a systematic method for comparative-historical linguistics, and the ways in which national activists in Eastern Europe took up these understandings, reproducing and transforming them in their appeals to proposed national communities. Starting with a survey of early Indo-European grammatical and linguistic studies, I draw on the methods of intellectual history to provide a better understanding of how languages and their histories were understood as objects of study, as well as an account of methods of classification, with an eye to understanding how this might shape both the specific terms and stakes of argument as these categories were taken up and publicized by nationalists in specific national contexts. I then discuss the Hungarian and Lithuanian cases in turn, focusing first on the linguistic and national ideas embedded in debates and participant writings before turning to their interaction and their subsequent reception in the popular press. The project concludes with a section on the use of linguistic classification in both contexts to differentiate the nation from internal and external others. This linguistics-grounded historical approach allows a specific account of the role of language in nationalism and the rise of national identity that reinforces previous general accounts, as well as offering a model for understanding analogous debates across Eastern Europe, such as the disputed national linguistic status of Ruthenian and Yiddish or the origins of Romanian and vernacular Greek.