Folksonomies have the potential to add much value to public library catalogues by enabling clients to: store, maintain, and organize items of interest in the catalogue using their own tags. The purpose of this paper is to examine how the tags that constitute folksonomies are structured. Tags were acquired over a thirty-day period from the daily tag logs of three folksonomy sites, Del.icio.us, Furl, and Technorati. The tags were evaluated against section 6 (choice and form of terms) of the National Information Standards Organization (NISO) guidelines for the construction of controlled vocabularies. This evaluation revealed that the folksonomy tags correspond closely to the NISO guidelines that pertain to the types of concepts expressed by the tags, the predominance of single tags, the predominance of nouns, and the use of recognized spelling. Potential problem areas in the structure of the tags pertain to the inconsistent use of the singular and plural form of count nouns, and the incidence of ambiguous tags in the form of homographs and unqualified abbreviations or acronyms. Should library catalogues decide to incorporate folksonomies, they could provide clear guidelines to address these noted weaknesses, as well as links to external dictionaries and references sources such as Wikipedia to help clients disambiguate homographs and to determine if the full or abbreviated forms of tags would be preferable.