TRT - Structure

The TRT is organized into a set of 21 hierarchies, called facets.  Each facet consists of one Top Term that represents a principle concept within the TRT.  Below each Top Term are terms that are both closely related and subordinate to it.

Terms within the TRT are labeled using a notation system.  The system identifies the Top Term using a capital letter, and uses lower case letters to indicate subordinate levels in the hierarchy, resulting in a unique notation for each term in the thesaurus.

Tree graphical image showing TRT relationships

Figure 1: Example of the TRT Structure

Terms within the TRT are linked through their semantic relationships to one another.  The semantic connection between terms can generally be defined as one of three relationship types: hierarchical, equivalent, or associative.

Hierarchical relationships

Hierarchical relationships are based on levels of superordination and subordination, where subordinate terms represent members or parts of the superordinate term.  Hierarchical relationships are expressed as broader terms (BT) and narrower terms (NT).

Tree graphical image showing TRT hierarchical relationship

Figure 2: Example of the TRT hierarchical relationship

Underground structures are a type of Specialized facility, while Underground parking garages in turn are a type of Underground structure.

Equivalence relationships

It is not uncommon for one concept to be represented by several different terms.  Because of this, within the TRT, one preferred term is selected for each concept.  Terms that are synonyms or near-synonyms to the preferred term, those that have variant spellings, or those that represent a focus that is too narrow for the scope of the TRT are linked to the preferred term.  In the full display of the preferred term, these non-preferred terms are referred to as Use For terms (UF).

Tree graphical image showing TRT use for relationship

Figure 3: Example of the TRT non-preferred relationship

Ground transportation is the preferred term, and both Land transportation and Surface transportation are non-preferred terms for this concept.  Any searches that are performed on either of these terms will refer the searcher to Ground transportation.

Tree graphical image showing TRT second use for relationship

Figure 4: Another example of the TRT non-preferred relationship

Given the TRT’s focus as a transportation resource, the addition of Taverns as a preferred term, would be a far too specific term.  However, it is possible that indexers or researchers might search for the term.  Therefore, it is added as a non-preferred term within the broader topic of Drinking establishments, which is an established term within the TRT.

Associative relationships

Associative relationships exist between terms that are neither hierarchical nor equivalent, but are nevertheless semantically related to one another.

Within the TRT, there are two types of associative relationships: related terms, and cross-references.

Related terms

Related terms (RT) are terms that are siblings within the same branch of the hierarchy, and which therefore, possess the same superordinate term.

Tree graphical image showing TRT associative relationship of related terms

Figure 5: Example of the TRT associative relationship of related terms

Fuel compsition has two siblings or related terms, Alcohol fuels and Biomass fuels. All three terms are the children of Fuels.


Cross-references point toward terms with associative relationships that exist in separate branches of the hierarchy.

Tree graphical image showing TRT associative relationship of cross reference

Figure 6: Example of the TRT ssociative relationship of cross reference

Although they have different parent terms, the topics of Vehicle operations, and Occupant protection devices relate to the concept of Vehicle safety, and are therefore added to that term as cross-references.  Note that although they may be of interest in the topic of Vehicle safety, Vehicle operations and Occupant protection devices are not related to one another.  Therefore, there are no cross-references between these two terms.