This paper deals with the oldest changes in the Proto-Slavic language (recorded in Old Church Slavonic) leading towards Štokavian dialectal stratification. The body of research supporting this article consists of forty-six charters, whose creation (whether as an original or a transcription) tied them to Raška, Bosnia, Hum, and Dubrovnik – a relatively wide but well-connected area. They were written in Cyrillic from the 12th to the 14th century. The corpus is taken from Ljubomir Stojanović (Stare srpske povelje i pisma. Knjiga 1: Dubrovnik i susedi njegovi, 1929), who used the texts from Dubrovnik’s archive. The fundamental assertion of this paper is that the charters present relative regional and temporal linguistic homogeneity, and that linguistic stratification is negligible considering the regions and period in our area of focus. Therefore, we moved our focus from the national-philological perspective to shared processes, which outweigh the differences between eastern and the western Štokavian and more recent national contexts. Taking the variety in the spatial origin of particular charters into account, any findings can be retrospectively applied to any South Slavic philology. For a full assessment of whether we are dealing with originals or copies made in Dubrovnik (which is a very important level of language analysis, considering all the changes that might have been made either intentionally or accidentally during the transcription), it would be necessary to make a complete palaeographic analysis based on comparative methods. Although greater differences might be expected considering the broad geographical area, which today hosts a multitude of dialects, the differences are almost negligible on the level of verb morphology. On the other hand, there are just enough contrasts to motivate a detailed analysis of this corpus, or even a somewhat larger one, at all grammatical levels. After the verb analysis, we confirmed our assertion about the early homogeneous development of the Štokavian dialect in the area covered by the corpus of texts through the statistical analysis of tokens from every mentioned region. The conclusion is that, between the 13th and 14th centuries, there were no significant differences, and that no notable language distribution between regions/countries could be recognized. In comparison with the earlier stage (in the context of the time when the charters are written) of this body of language recently separated from the Proto-Slavic community (12th century), the appearance of the language features that would lead to Štokavian specificity is quite obvious. This paper contributes to an understanding of the relationships between the eastern and western Štokavian dialects – relationships that developed out of the West-South Proto-Slavic language (together with Slovenian, Kajkavian, and Čakavian) in a different rhythm of extraction, i.e. through a different period of coexistence. Innovations in verb features (compared to Old Church Slavonic) belong to both eastern and western Štokavian regions, and do not present solid evidence of characteristic differences between them.