Usage also changes over the years. The modern connotation of the word Environment in popular usage has a much narrower focus than in its former usage, where it referred not only to the natural environment, but also to any physical environment. Because of this ambiguity, LC has never established Environment as a heading, preferring to use it as a reference to other more specific headings. The word Ecology in common usage has a meaning similar to that of Environment, whereas in the scientific community it has a much more specific meaning, which it retains in LC practice. It is sometimes as difficult to predict what heading will be assigned to a work as it is to predict what search words are used.
Persistence is required of the user and the reference librarian.
One problem with both assigning and using the LC subject headings is determining what the heading means. LC assigns the narrowest possible subject heading to a work so as to be as specific as possible, yet many terms overlap or are very closely related. The scope can often be gathered by examining the subject authority record and the cross-references and notes it contains. A full MARC format authority display revealing the headings broader or narrower terms is often helpful in determining more precisely the scope of a term. LC also sometimes provides a scope note.