Based on the findings of Arnstadt’s archivist Karl Müller, Kurt Hermann Frickel’s Bach genealogy, and other sources, the article clarifies the identities of Bach’s two cousins: Barbara Catharina Bach (1680–1709) and Barbara Katharina aka Catharina Barbara Bach (1679–1737). In Bach studies of the late twentieth century and on, the identities of these two cousins merged into one. This confusion led to two erroneous conclusions: 1) the misidentification of the girl Bach accompanied to her home and who witnessed Bach’s famous street brawl with the student Heinrich Geyersbach; and 2) the misidentification of this person as a sister of Maria Barbara, whom Bach eventually married. The author untangles this confusion and establishes the identity of Bach’s companion on 4 August 1705.