An obvious way in which the structure of arrays can be improved is by displaying any hierarchical relationships between concepts. We have already looked at the nature of the broader/narrower term relationship in some detail in Chapter 13, together with a large number of examples of different kinds of relationship. Consideration of the hierarchy in this chapter is mainly of the particular needs of the animals facet, so, although some general principles are discussed, this is mainly a demonstration of putting the theory into practice in a specific situation.

Hierarchical structure

Compared with an enumerative, pre-coordinated system, a faceted structure is usually a rather flat structure, with limited levels of hierarchy. This is because it doesn't contain many complicated compounds. Nevertheless some degree of hierarchy will exist and should be represented in the systematic structure. Concepts that exist in a superordinate/subordinate or broader/narrower term relationship need to be brought together in the linear order so that these relationships can be properly displayed.

The recognition of these broader/narrower term relationships seems to be intuitive, and even novice thesaurus compilers experience little difficulty with this part of the process. This is particularly so when the terminology can be easily understood by a lay person, although in the case of a technical subject, some help may be required from a dictionary or encyclopaedia. Our animal terms are relatively straightforward to interpret, to some extent because of the decision to use vernacular names. Despite that, there are a few challenging terms (because no vernacular equivalents exist) and I needed the aid of a dictionary to discover that ‘ratites’ is the containing class for emus, ostriches and rheas.

Representation of hierarchy

We normally represent hierarchy in the systematic display by indenting a subordinate concept to its containing class, as follows:

This visual representation of subordinate classes will enable us to identify the broader and narrower terms very easily when we come to convert the taxonomic structure to thesaurus format. Because all the terms in a facet are of the same general kind (i.e. all entities, or processes, or products) there will not usually be any problem in establishing whether the relationships are genuine hierarchical relationships, of a thing/kind or whole/part nature, since they cannot really be otherwise. In the hierarchy