The word “taxonomy” is widely used these days in discussions about organising knowledge or information, especially information in electronic form such as that on the Internet. There is no generally accepted definition of what taxonomies are, though the word is usually used to refer to alphabetico-classed schemes of subject headings. These are built on fundamental principles, which have been more fully developed in their application to thesauri and classifications. By recognising that all these kinds of “controlled vocabularies” are just variations on a theme, built from the same components, it is possible to create consistent and complementary tools to provide for the different approaches which users may need.