A
bibliographic index is a
bibliography, an aid to search the literature of, for example, an academic field or discipline (example:
Philosopher's Index), to works of a specific literary form (
Biography Index) or published in a specific format (
Newspaper Abstracts), or to the analyzed contents of a serial publication (
New York Times Index). Indexes of this kind are issued in print
periodical form (issued in monthly or quarterly paperback supplements, cumulated annually), online, or both. Since the 1970s they are typically generated as output from
bibliographic databases (whereas earlier they were manually compiled using
index cards). Citations are usually listed by author and subject in separate sections, or in a single alphabetical sequence under a system of authorized headings collectively known as
controlled vocabulary, developed over time by the indexing service.