The large number of citations in many catalogs and databases requires one to limit otherwise broad or general searches in order to retrieve a manageable and pertinent number of results. Conversely, overly narrow search terms can return too few results. One way of solving both problems is to use Boolean operators (AND, OR, NOT), which allow one to limit or expand searches depending on his or her needs.
For example, a search for Resnais AND Holocaust will return items that contain both "Resnais" and "Holocaust":
Resnais OR Holocaust returns items that contain either "Resnais" or "Holocaust" or both:
Resnais NOT Holocaust returns items that contain "Resnais" but not "Holocaust":