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.
Article indexes are a type of bibliography. The purpose of bibliography is to list documents, usually published documents like books and articles. This type of bibliography is more accurately called "enumerative bibliography". An enumerative bibliography will attempt to be as comprehensive as possible, within whatever parameters established by the bibliographer.
Think of a bibliography as a guide to the source base for a specific field of inquiry. A high quality bibliography will help you understand what kinds of sources are available, but also what kinds of sources are not available (either because they were never preserved, or because they were never created in the first place).
For more information on the role of bibliography in historical research, see our guide to Bibliography and Historical Research.
Bibliographies can be as short as a few pages, or as long as several hundred volumes. Bibliographies can also be published as databases, and these are the bibliographies that are often called "article indexes" or "indexing and abstracting services" because they index the contents of journals.
There are many article indexes for finding periodical articles. 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 database for identifying journal articles in American history is:
Although America: History and Life is considered the most "important" article index for research in American history, there are several other very important article indexes, any of which might be crucial for research depending on the focus of your research:
Most article indexes have been digitized and are now available as databases, but not all:
1. D.E. Davinson, The Periodicals Collection: Its Purpose and Uses in Libraries (London: Andre Deutsch, 1969), 38.