Editor's note: This book was never intended to be a textbook on how to build taxonomies. But if you need a crash course or refresher, Bob's chapter introduces all the key points, using examples of everyday things like beverages and dog food. Sometimes the jargon of taxonomies can seem esoteric, yet it exists to make taxonomies make sense and thus work for their users. Semantics is about understanding the underlying meanings of the words in a taxonomy so that they can be organised logically – few people other than Bob can make this under-standable as well as fun.

Introduction

The topic of ‘Taxonomy’ is both strictly and loosely defined. This is recursive (being the category of things about categories), which is a pleasure.

Taxonomy, broadly speaking, is the art and science of categorisation. Many controlled vocabularies that fall under the topic of ‘Taxonomy’ are not taxonomies: authority files, which are flat lists (that is, they have no hierarchy), such as countries, people or organisations, are closely related to taxonomies and are often included in the discussion. On the other extreme, ontologies (which are seriously beyond the scope of this chapter), which may include taxonomies as a part of their structure, have become part of the world of taxonomy.

Frequently, taxonomies are used to structure groups of subjects, topics or categories: organising products on a website or structures of topics to organise content (think the Dewey Decimal System or a topical browse on a website) into broader and narrower categories so that people can find things in large repositories of information. These categories are objects in an information system.

A taxonomist is therefore a kind of Gorgon. Like the mythical beast from Greek mythology that turned people into stone, a taxonomist turns subjects into objects.

The problem, of course, is that language is fluid and the meaning of words changes over time (and in contexts!). The science of taxonomy is, correspondingly, an inexact and tricky one. Deciding which categories are relevant and what they are called has practical (what are the branches of science?), philosophical (what does it mean to name something?) and ethical implications (who gets to say how many genders are included on your web form?).