I argue that the availability of each marker to convey each of these readings at different points in time ultimately responds to the structure of the meanings that participate in the change, and to communicative and cognitive pressures that are reflected in features of the contexts in which these markers are interpreted. To address the nature of these representations and processes, I present evidence from Spanish, a language with two markers –the (old) Simple Present and the (newer) Present Progressive– that alternatively convey the event-in-progress and the habitual reading in a many-to-many mapping. This distributional pattern points to a more complex stage in the grammaticalization path in which a full categoricalization of the system has not been yet achieved, but where the generalization of the newer marker seems to be already underway. Through a corpus study across different time periods, and with data from experimental tasks in different dialectal varieties, I show how specific communicative and cognitive constraints have triggered the recruitment of the progressive marker, its categoricalization for the event-in-progress reading, and its incipient generalization into the more general Imperfective domain. Altogether, the patterns observed across times and dialects are consistent with a model of semantic variation and change embedded in a communicative system, subject to isolable contextual factors, and observable through experimental methodologies and careful examination of textual records.