This paper explores the relationships between natural
language lexicons in lexical semantics and thesauri in
information retrieval research. These different areas of
knowledge have different restrictions on use of vocabu-
lary; thesauri are used only in information search and
retrieval contexts, whereas lexicons are mental systems
and generally applicable in all domains of life. A set of
vocabulary requirements that defines the more concrete
characteristics of vocabulary items in the two contexts
can be derived from this framework: lexicon items have
to be learnable, complex, transparent, etc., whereas the-
saurus terms must be effective, current and relevant,
searchable, etc. The differences in vocabulary proper-
ties correlate with two other factors, the well-known
dimension of Control (deliberate, social activities of
building and maintaining vocabularies), and Syntagma-
tization, which is less known and describes vocabulary
items’ varying formal preparedness to exit the thesau-
rus/lexicon, enter into linear syntactic constructions,
and, finally, acquire communicative functionality. It is
proposed that there is an inverse relationship between
Control and Syntagmatization.