The following sites offer guides to citing primary sources in the Chicago style:

Library of Congress

University of Wisconsin-Milwaukee

Citation at a Glance: Primary Source from a Web Site (Diana Hacker)

Primary Sources at Yale defines and explains the importance of primary sources along with a series of questions for evaluating documents.

The National History Day Research Roadmap provides a good discussion about the definition and use of primary sources.

Five Criteria for Evaluating Web Pages from Cornell University offers an excellent guide for evaluating primary sources. 

A bibliography from a good secondary source is a great way to identify primary sources.

Use all variations of a keyword.

Words change over time. Today we say Civil War but, in the past, it has been called the

• American Civil War
• War between the States
• War of Secession
• War of the Rebellion

If you searched only the term Civil War, you might miss valuable documents written during a different era.

The Library of Congress Authorities and Vocabularies thesaurus will help you find variations of keywords and subject headings.

Video produced by the University of California, San Diego's Social Sciences & Humanities Library.

"Primary sources provide first-hand testimony or direct evidence concerning a topic under investigation. They are created by witnesses or recorders who experienced the events or conditions being documented. Often these sources are created at the time when the events or conditions are occurring, but primary sources can also include autobiographies, memoirs, and oral histories recorded later. Primary sources are characterized by their content, regardless of whether they are available in original format, in microfilm/microfiche, in digital format, or in published format." -- Yale University

Examples of primary sources include

To find primary sources, try using these terms in keyword searches: