The aim of this thesis is to, through an empirical study of the social tags in the City of Stockholm libraries’ OPAC with critical discourse analysis, examine the social tags’ linguistic design, usability, and communicative potential – all in relation to the context. It is a linguistic study and it is important to take into account that the tags are extremely short texts – single-word-texts. My starting-point is that the tags are a form of utility texts. What I find is that a clear majority of the tags are nouns and function as index terms, but that there are differences compared to authorized index terms. The study shows how the tag-authors solve their contextual premises by for example word-compositions. I find that a balance is important between specific and general as the tags lack a parent-child or sibling semantic relationship. Compared to traditional knowledge organization, there is a consistent feature of subjectivity in the tags. I see a fourth type of organization in addition to cataloguing, classification, and indexing – the personal comment. In the retrieval, the tags are best suited for browsing. I also find that the social tags could function as a complement to a controlled vocabulary, but not replace it.