The exploration of spoken language in diachrony, with a particular focus on the individual linguistic varieties of the French diasystem, has increasingly been addressed in Romance linguistics. In particular, the necessity of more thoroughly capturing the historical development of non-standard language varieties has been repeatedly emphasized. This work systematically analyzes the language of the Agréables Conférences (1649-1651) in comparison with other contemporary text types and grammars relevant to français populaire, with the aim of gaining new insights into the French substandard in the 17th century and its relationship to bon usage and to present-day varieties.