There are four types of Controlled Vocabularies:
Lists of terms with no semantic relationships. For example a list of values for a drop-down menu.
Terms linked together with equivalent synonyms (our kitty cat image below).
Hierarchal lists (with broader and narrower categories) or faceted lists which also might include synonyms.
The most complex form which combines all three types of semantic relationships (synonyms, hierarchy and associations such as: used for, broader term/BT, narrower term/NT, related term/RT).
As you move from left to right we see that the structural complexity increases in the types of semantic relationships present (equivalence, hierarchical and associative).
Not everyone agrees on the order represented above:
Morville and Rosenfeld (The Polar Book/Information Architecture for the World Wide Web pp. 194-201) put things in a slightly different order (synonym rings then value lists) which I’ve never really quite understood their reasoning behind. However, I prefer the method used by Stephen J Miller in Metadata for Digital Collections. I highly recommend you buy his book now. Although its primarily focused on Digital Collections its one of the best books to read if you are trying to get your head around metadata.
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Metadata and Taxonomies are my thing. I spend an awful lot of time drinking coffee and having existential dilemmas on how to categorize what the hell I do for a living.