This book argues that languages are composed of sets of 'signs', rather than 'strings'. This notion, first posited by de Saussure in the early 20th century, has for decades been neglected by linguists, particularly following Chomsky's heavy critiques of the 1950s. Yet since the emergence of formal semantics in the 1970s, the issue of compositionality has gained traction in the theoretical debate, becoming a selling point for linguistic theories. Yet the concept of 'compositionality' itself remains ill-defined, an issue this book addresses. Positioning compositionality as a cornerstone in linguistic theory, it argues that, contrary to widely held beliefs, there exist non-compositional languages, which shows that the concept of compositionality has empirical content. The author asserts that the existence of syntactic structure can flow from the fact that a compositional grammar cannot be delivered without prior agreement on the syntactic structure of the constituents.