Grammaticalization by changing co-text frequencies. Or why [BE Ving] became the 'progressive'

While the 'progressive' construction [BE Ving] (He was playing tennis when Jane came in) has been studied extensively both diachronically and synchronically, studies of its functional development tend not to extend further back than Early Modern English. This paper draws attention to the functional changes [BE Ving] goes through already in Middle English, whose analysis sheds new light on the principles of early grammaticalization. To understand the observed changes, all uses of [BE Ving] are considered, not only those that have a clear verbal and aspectual function. During Middle English, important changes occurred in the frequencies of the various co-texts of [BE Ving]. They involve the increase in backgrounding adverbial clauses, which leads to the semanticization of ongoingness, a feature that was initially only associated with [BE Ving] by pragmatic implicature. The outcome is grammaticalization by co-text: co-textual changes paved the way for the acquisition of progressive semantics in [BE Ving] itself.