Project B06 investigates and models the interaction between language change and register knowledge: how is change driven, in which direction, and is it propelled from above (by higher registers) or below (by lower registers)? By studying one of the most common words, ‘and’, project B06 focuses on extinct Indo-European languages which had two such words for conjunction, e.g. Latin que and et. B06 already carried out a preliminary study on the history of Latin which shows that the choice of conjunction marker correlates with register. B06 will build formal models of the interaction between language change and register on the basis of the Latin data. B06 also plans to expand the empirical scope by investigating several other Indo-European families, and their relative historical developments. The broader empirical scope will allow B06 to test and adjust its model and contribute to knowing more about how register acts as a driving force of language change.