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 two most important databases for identifying journal articles in history are America: History and Life, and Historical Abstracts. The two databases complement each other, and you will choose one or the other (or both) depending on the region you are researching:
Although America: History and Life, and Historical Abstracts are the two most "important" article indexes for historical research, there are several other very important article indexes, any of which might be crucial for research depending on the focus of your research:
Not necessarily scholarly, the collections here include popular periodicals ("magazines"), trade journals, and serious (but not scholarly) periodicals like the Economist.
Below are the major digital collections that reproduce alternative periodicals. In some cases, these periodicals are embedded in larger collections of mixed document types (for example: The Sixties: Primary Documents and Personal Narratives).
1. D.E. Davinson, The Periodicals Collection: Its Purpose and Uses in Libraries (London: Andre Deutsch, 1969), 38.