The controlled vocabulary (or thesaurus) details the relationship of terms with one another. There are three kinds of relationships. Equivalency: where terms refer to the same concept; for example, cat and feline can be considered as equivalent. Equivalency is the kind of connection between terms in a synonym ring or between preferred terms and variant terms. Hierarchical: where terms are hierarchically arranged from Broad Term (BT) to Narrow Term (NT). The term at the top is called the Top Term (TT) Associative: where terms are associated with each other but are not hierarchical or equivalent. They are said to be Related (RT).