This article is concerned with the characteristic features that define medieval merchants’ letters. It explores systematically a collection of merchants’ letters from early 16th-century Finland, the so-called Scheel’s collection, which is unique in medieval northern Europe. Scheel’s letters show surprising conformity. They followed the conventions of letter writing, including their content, form, and style. Scheel’s letters were employed primarily as operative instruments in the organizing and controlling of transactions, reflecting similar functions to other European merchants’ letters. In line with other European correspondences, they also included most of the components of the rules of letter writing.