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:

1. Publication of current information. Periodical publishers and book publishers operate under very different business models, and the former are able to publish new information far more quickly and inexpensively than the latter.
2. Publication of information that does not lend itself to publication in book format, either because even a full treatment of the subject would be too short to warrant publication as a book, or because the audience for the information is so specialized that publication as a book would represent too great a market risk for a publisher to assume.¹

There are dozens of types of periodicals. Four important types are described below:

• Scholarly Journals. Often called journals. Intended for academic audiences. Many scholarly periodicals are published by university presses, learned societies, and other not-for-profit publishers, but others are published by commercial (or "for-profit") publishers like Routledge, Brill, De Gruyter, Springer, Brepols and, Sage. Not all scholarly periodicals are peer reviewed.
• Popular Magazines. Often called magazines. Almost always published by commercial ("for profit") publishers, and often cease publication when they become unprofitable for the publisher. Intended for popular audiences, but can cater to smaller audiences (like hobbyist magazines) as long as the audience is segmented such that the magazine can still be profitable. Pornographic magazines, like the gay fetish magazine Bound and Gagged, are also technically commercial, and form part of the Library's periodical collection.
• Trade Papers. Often called trade journals or trade newspapers. Intended for members of a profession (Chronicle of Higher Education), occupation (Railway Carmen's Journal), or industry (Hollywood Reporter). They often resemble newspapers in frequency of publication (weekly and even daily) and appearance (printed on large format, inexpensive paper, with no cover).
• Newsletters. Newsletters are typically published by an organization, and report on the activities of that organization. The intended readers of a newsletter are members of the organization, people who have an interest in the organization, or people whom the organization hopes will have an interest in it. A newsletter might also publish articles they expect will be of interest to its members, but that are not, strictly speaking, about the organization itself. Related to both newsletters and trade papers (q.v.) are circulars and bulletins. A Circular is a a synecdoche for "circular letter," basically a newsletter that is distributed among a group (or "circle") of people who share a common interest. A Bulletin would typically be a publication of authoritative information (in other words, issued by a recognized authority) and intended for wider distribution, often to the press. In modern usage all three are often used interchangeably.
• Alternative Press. Has an acknowledged political bias that is considered to be outside the "mainstream".  Alternative periodicals are usually not expected to turn a profit. They are intended to motivate readers to action, or form coalitions from like-minded people. They are often low-budget newsletters, but can also be expensively produced magazines or organs of societies and special interest groups. Alternative newspapers and periodicals are sometimes mistakenly referred to as "underground newspapers." Strictly speaking, an underground publication is one that is published secretly--in other words, the place of publication and identity of the publisher are not disclosed. Most alternative newspapers and periodicals in library collections do not meet this criterion.

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.

In the field of history, secondary sources are the scholarly "conversation" taking place about the past.

Secondary sources can include scholarly books, articles, and essays (both analyses by contemporary scholars as well as older scholarly analyses), surveys, criticism, comparative studies, reference sources, and works on theory and methodology.

To identify secondary literature, you can do subject searches in the Library Catalog to find books or subject searches in article indexes/databases to find articles; article databases may list books as well as articles from journals. You can also consult bibliographies.

Other ways of finding relevant secondary sources include looking for review essays, in which a historian who specializes in the subject analyzes recent scholarship and looking for historiographical treatments of the topic published as chapters in collections, journal articles, or even monographs.

Learn more about scholarly communication (AKA secondary sources):

Use article indexes to identify articles from journals and periodicals. When you search an article index, you are searching bibliographic records that describe articles; you are not searching the actual articles. For many of the citations you retrieve in an article index, the database will also provide direct access to the article.

The key distinction between an article index and a full text database is that, in an article index, not every article you identify will be available online. In a full text database, on the other hand, all the articles are available online. Most article indexes began as serialized bibliographies (see explanation under Reference Sources > Bibliographies, Catalogs, and Guides), and many still contain the word "bibliography" in their titles (e.g. Bibliography of British and Irish History).

Article indexes will also contain records for other types of documents, such as dissertations, books, book chapters, conference proceedings, book reviews, and more.

The two main article databases for history are Historical Abstracts and America: History and Life (see their entries below). One or the other of these databases is usually the best starting place to search for scholarly articles in English on topics in history.

Searching these databases directly, rather than through Easy Search, offers you more search options.

IMPORTANT: If you find a journal article that you want to read, but that the library does not own, you can request a copy through Interlibrary Loan. Journal articles are usually delivered very quickly (1-2 days, sometimes the very same day). You can also request books through Interlibrary Loan, though the wait period is a little longer (about a week).