Abstract In this article, we examine, quantitatively and qualitatively, Dutch business letters written by merchants, focusing on formulaic phrases and French suffix borrowing. The letters are part of the Prize Papers, the collection of documents confiscated by the English during times of warfare. We compare these business letters to private letters from the same source, and address the question whether the business letters constitute a particular register. The business letters, written by upper and upper-middle ranked male merchants from the seventeenth century, appear to differ from private letters written by people with a similar social profile. Frequent formulae highly characteristic of seventeenth-century private letters occur to a much lesser extent, if at all, in the business letters. Other formulae, some including Romance origin items (Laus deo, saluut, cordiale), occur more frequently in the business letters. The frequency of verbs with the French-origin suffix -eren is found to be higher in the business letters than in the private letters. Functional explanations are presented for these register differences: formulaic language is correlated with writing experience, and the use of French-origin items may be connected to the common vocabulary used in international trading networks. Similarities in phrasing are also revealed in a qualitative analysis of examples of private and business letters by the same writer. This finding demonstrates how similar private and business letters often were, thus challenging the idea of distinct registers.