This fonds consists of Thiessen's correspondence and various materials he collected while compiling his books. In addition to the correspondence and notes, there are three card files: Male Thiessens (and variant spellings), female Thiessens, and an alphabetical card index which apparently includes all Mennonites he found who were born before 1820 or baptized before 1840. The first file folder contains some introductory and explanatory notes by Hermann Thiessen. Some of the people he corresponded with and received information from were Katharina Thiessen, Fritz van Bergen, and Anna Andres.