Indexing with the Thesaurus

The Government of Canada Core Subject Thesaurus is a post-coordinated controlled vocabulary. Unlike a pre-coordinated vocabulary, in which concepts are combined at the time of indexing to form more complex ideas, a post-coordinated vocabulary mainly proposes unitary concepts which are combined by the searcher only at the searching stage.

Compliance with the following indexing rules will ensure a greater uniformity in assigning indexing terms (preferred terms). By following the rules, all the Government of Canada Core Subject Thesaurus users will represent the same concepts the same way in the "Subject" metadata element.

For more information on how to index Government of Canada information resources in accordance with the Treasury Board of Canada standards, please consult the Government of Canada Metadata Implementation Guidelines for Web Resource Discovery.

Indexing rules

1. Specificity

It is important to choose the most specific terms available when describing the subject of a document. Thus, a resource about aquatic birds should be indexed with the preferred term "Aquatic birds" rather than with the broader terms "Birds" and "Aquatic animals".

Similarly, a document about birds in general should be indexed with the preferred term "Birds" rather than with the broader term "Animals" or the narrower terms "Aquatic birds" and "Migratory birds".

As a general rule, a document should not be indexed with both broader and narrower terms from a same hierarchical string. However, it would be necessary, for example, to index a resource about birds in general, which also included a substantial section on migratory birds using both "Birds" and "Migratory birds" as preferred terms.

The semantic structure of the thesaurus is very useful for identifying the appropriate preferred terms. Relationships between terms (broader term, narrower term and related term), as well as scope notes, help to define the meaning of the preferred terms, to suggest other terms that might be relevant, and therefore are aids to selecting the most adequate terms.

2. Exhaustivity

Several preferred terms are often necessary to fully express the subject of a document. For example, to index a site about transportation of toxic waste and other dangerous products by rail or by truck will be represented by the following set of preferred terms:


Hazardous materials transport
Rail transport
Road transport
Toxic substances

As far as possible, every essential concept dealt with in a document should be represented by a preferred term.

3. Consistency

It is essential to use the form of the preferred terms as presented in the Government of Canada Core Subject Thesaurus when using them in the "Subject" metadata element.

As a general rule, only the first letter of an indexing term is capitalized. Conventions are also followed in the Government of Canada Core Subject Thesaurus regarding the use of singular and plural forms. In English, count nouns (which are subject to the question "how many?") are generally in the plural form, while non-count nouns (which are subject to the question "how much?"), as well as abstract concepts, are in the singular form. In French, most terms are in the singular form.

4. Indexing language

Documents in English must be indexed with the English version of the thesaurus and documents in French with the French version. Bilingual documents should be indexed in both languages.