Like the term “taxonomy” the term “thesaurus” has been used to describe all kinds of subject classification structures, though for thesauri there are two ISO standards describing their structure. [ISO2788] describes monolingual thesauri, while [ISO5964] is for multilingual thesauri. We will here discuss thesauri as they are defined in the ISO standards, while noting that in practice many users extend the structure somewhat, and in some cases the term is applied to structures differing substantially from what is described here.
Thesauri basically take taxonomies as described above and extend them to make them better able to describe the world by not only allowing subjects to be arranged in a hierarchy, but also allowing other statements to be made about the subjects. [ISO2788] provides the following properties for describing subjects.
- Short for “broader term”, refers to the term above this one in the hierarchy; that term must have a wider or less specific meaning. In practice some systems allow multiple BTs for one term, while others do not. (There exists an inverse property known as NT, for “narrower term”, which is implied by the BT.)One could say that taxonomies as described above are thesauri that only use the BT/NT properties to build a hierarhcy, and don’t make use of any of the properties described below, so it could be said that every thesaurus contains a taxonomy.
- This is a string attached to the term explaining its meaning within the thesaurus. This can be useful in cases where the precise meaning of the term is not obvious from context. “SN” stands for “scope note”.Since users often use “XTM” when they mean “topic maps”, it would be useful to add a scope note to XTM saying something like “The standard XML interchange format for topic maps. When discussing topic maps in general, and not just the format specifically, use the term ‘topic maps’.”
- Refers to another term that is to be preferred instead of this term; implies that the terms are synonymous. (There exists an inverse property known as UF.)For example, on “topic navigation maps” we could put a “USE” property referring to “topic maps”. This would mean that we recognize the term “topic navigation maps”, but that “topic maps” means the same thing, and we encourage the use of “topic maps” instead.If we do this we would also have a “UF” property on “topic maps” referring to “topic navigation maps”, since this is implied by the “USE” relationship.
- Is short for “top term”, and refers to the topmost ancestor of this term. The term at the other end of this property is the one that would be found by following the “BT” property until you reach a term that has no “BT”. This property is strictly speaking redundant, in the sense that it doesn’t add any information, though it may be convenient.
- Short for “related term”, refers to a term that is related to this term, without being a synonym of it or a broader/narrower term.For “topic maps” we could use this to indicate that “subject-based classification” and “ontologies” are terms related to “topic maps”.
In short, thesauri provide a much richer vocabulary for describing the terms than taxonomies do, and so are much more powerful tools. As can be seen, using a thesaurus instead of a taxonomy would solve several practical problems in classifying objects and also in searching for them.