The particle bylo, which dates back to the Old and Middle Russian Pluperfect, exhibits interesting trajectories of gradual evolution. Its combinatory properties, semantic interpretation, and prosodic status have all undergone a marked change. The form is prominent also in a broader typological context as an instance of “frame past” markers, common both for Slavic and non-Slavic languages. Eighteenth-century texts, examined from the Russian National Corpus and other sources, offer an intermediate link between the Old/Middle Russian system and the modern use of bylo. The construction was not yet thoroughly lexicalized and allowed for such semantic types of predicates such as modal verbs or accomplishments, including irreversible ones. It was synonymous with another parenthetical particle byvalo (originally making part of serial verbal constructions) in some contexts, whereas the constructions with byvaloexhibited some particular combinatory features in their own right.