Based on an S-curve model of language change, the study establishes that overt markers of definiteness were first made available to identifiability-based definites, then to inclusiveness-based definites, quantitative generics and unique referents. The overt markers of indefiniteness followed suit, separating indefinites from non-specifics and typifying generics. This progression of definiteness was directed by variables such as person, animacy, gender, number and noun-class, and started in contexts in which definiteness closely interacted with possessivity. Such an analysis leads to the realization that the two-dimensional S-curve model does not account for all language change and that there is a need for a three-dimensional model. It also demonstrates that, contrary to previous assumptions, there is continuity between the early Slavic marker of definiteness (long-form adjectives) and the Modern Bulgarian article. This discovery, in conjunction with geolinguistic arguments, sheds new light on the role that relations inside the Balkan played in the grammaticalization of Bulgarian definiteness.