As explained in the section on Documents, the periodical evolved from the book, and the reason is that the periodical filled two main needs that the printed book could not:
There are dozens of types of periodicals. Four important types are described below:
These distinctions are simply a method for classifying sources; and, like all classification schemes, it provides a method for quickly completing a task, in this case the task is drawing certain conclusions about the nature of a source. The conclusions you draw should not be your final judgment on the question of the source's value. Classification schemes often obscure as much as they reveal about whatever they are attempting to describe.
Neither source type ("scholarly" or "popular") definitively indicates the value or reliability of a source, but recognizing the difference can sometimes make it easier to predict the probability of a source's value and reliability. You still need to evaluate each source critically.
There are many article databases for finding periodical articles. These databases are often called article indexes, but they are essentially searchable bibliographies of journal articles organized by subject.
Because the Library does not subscribe to every journal, and because not all journals are digitized, and because not all digitized journals are available in a single collection, the article indexes provide the only efficient means of identifying relevant articles from across the widest possible range of periodical publications.
Most of these article indexes include a mixture of academic and popular sources (and remember that sometimes the distinction is not clear).
The principal article index for identifying journal articles in American history is America: History and Life:
Other article indexes useful for research in this course, depending on the focus of your research topic, include the following:
1. D.E. Davinson, The Periodicals Collection: Its Purpose and Uses in Libraries (London: Andre Deutsch, 1969), 38.