Once the information industry had run into the limitation of controlled but unordered lists of keywords, thesauri came into being. In general, a thesaurus is also a controlled vocabulary of keywords, but this time, we can relate those keywords to each other.
Commonly, the following relationships are defined in a thesaurus:
Broader and narrower terms in a thesaurus result in a hierarchical structure of the thesaurus. If the broader term is vehicle, then truck, car, motorcycle and bicycle are all narrower terms. Broader and narrower relationships are directional terms. If car is the narrower term of vehicle, it cannot also be a broader term of vehicle. And: all cars are vehicles, but not all vehicles are cars.
This type of relationship is non-hierarchical and bidirectional. That is, if the first term relates to the second term, the opposite is also true: the second term relates to the first.
When building a thesaurus, you may want to establish some guidelines for this type of relationship. In the above example, you might be tempted to add the relates to relationship between truck and car, because they are both vehicles. But this does not add anything at all. However, if you use this relationship to express that both have an internal combustion engine, this relationship becomes meaningful.
This type of relationship enables us to work with synonyms. Car and automobile are synonyms. In our thesaurus we want to include both terms, but since car is the preferred nomenclature we add the relationship Use/Use for to these two terms. It expresses: use the term car whenever you consider to use the term automobile.
This way all hierarchical (i.e., broader and narrower relationships) can be constructed between the preferred terms, and the synonyms are attached to the preferred terms.
A scope note clarifies the scope of a concept (term), in particular when a term can be interpreted too broadly or too narrowly. A scope note may also contain instructions for the person indexing documents with the thesaurus.
The illustration below represents a thesaurus for transport. We see how the different concepts of the thesaurus are connected by relationships. Starting at the Transportation concept, we see three broader/narrower relationships with Land, Water and Air. The concept Land has four relationships: with Car, Truck, Motorcycle and Bicycle. These relationships are also of the broader/narrower type. The concept Car has a further relationship with the concept Automobile. This is a Use / Use for relationship, which expresses that Car is the preferred term and Automobile the non-preferred one.