Based on a correspondence between Eugenio Coseriu (1921-2002) and Eli Fischer-Jørgensen (1911-2010), the article discusses the importance of Louis Hjelmslev (1899-1965) for the development of the theory of Coseriu. In a letter dated 1955, Fischer-Jørgensen agrees with Coseriu in his criticism of Hjelmslev’s neglect of substance in phonology, but she also makes some critical remarks on Coseriu’s account. Starting from Fischer-Jørgensen’s remarks, it will be shown that there is some ambiguity in Coseriu’s attitude towards Hjelmslev’s terms norm and usage. Moreover, it will be shown that there seems to be a slight change in Coseriu’s view on content substance. This way, the article aims to show that Fischer-Jørgensen played an important role as a critical mediator between Hjelmslev and Coseriu.