Media studies are still in search of a conceptual foundation on which their findings from the history of technology, cultural anthropology, and discourse history can be described and explained within their contextual framework. All previously proposed media concepts, such as McLuhan's concept of prosthetics, Baudrillard's concept of simulacra, or Luhmann's distinction between medium and form, have been too concrete, too one-sided, or too broad to formulate such a foundation. This work proposes to understand media as historically mutable repertoires of forms, through which semiotic processes are particularly highlighted and made accessible for reflection. Second-order media semiosis not only exposes semiotic processes but also obscures them: media present semiotic processes in a way that requires interpretation. In this double sense of the statement.