Louis Hjelmslev (1899-1965) was a Danish linguist whose ideas formed the basis of the Copenhagen School of linguistics. Together with Hans Jørgen Uldall, he developed a structuralist theory of language that he called glossematics, which further developed the semiotic theory of Ferdinand de Saussure.
Glossematics is a theory of language that attempts to describe the formal and semantic characteristics of language in isolation from sociology, psychology, or neurobiology, and has a high degree of logical rigour. Hjelmslev regarded linguistics – or glossematics – as a formal science, and his theory became influential in structural and functional grammar, as well as semiotics.
Deleuze and Guattari took up in particular Hjelmslev's distinction between "forms of content" and "forms of expression."