Abstract The present article demonstrates how research on confiscated late-seventeenth-century letters allows us to gain insight into linguistic practices of second and third generations of Dutch-speaking migrants who lived in the German city of Hamburg, in a predominantly Low German region. The historical background of the preserved Heusch correspondence, spoken and written communication in merchant circles, and foreign language learning will be discussed. Apart from examining features such as epistolary formulae, ellipsis, and code switching, the question is also addressed of the degree to which interference from Low (or High) German is found. An analysis of the letters reveals both adoption of the Low German reflexive pronoun sick and a diverse pattern of using the relative particle so , which is shown to be a clear case of adopting (and maybe even extending) a supraregional German relativisation strategy.