Accordingly, Hjelmslev presented, as the key figure of Copenhagen School in the 1930s, a formal linguistic fundament, which was later known as glossematics (the double duality of the linguistic sign). He formulated his linguistic theory together with Hans Jørgen Uldall as an attempt to analyse the expression (phonetics and grammar) and the meaning of a language on a coherent basis. He assumed that language wasn't the only instrument of communication (cf. the communication of deaf-mutes), and he was interested in a general theory of the signs of communication, semiotics or semiology.