Image source: Reasonist. Used with permission.
Citing your references, or the sources of information that you use in your research, is critical for a number of reasons. Citing references:
shows you've incorporated other scholarly research into your work;
gives a record of the sources you've used in your research;
provides the reader with valuable information, indicating where to go to find further information;
extends professional honesty and courtesy. Writers have a responsibility to indicate when they have used someone else's ideas or words.
When citing a reference or compiling a bibliography, there are many style choices. Check with your professor to see which style (APA, MLA, etc.) you should use.
Writers use citations to acknowledge the sources of information they've used and to avoid plagiarism. A citation is the information needed to locate source material.
What does Chicago say about citing archival materials?
Citations in footnotes are usually to a specific item. The first element in a bibliography is usually either the collection in which the item may be found, the author, or the depository. (CMS16, 14.233)
“Citations of collections consulted online […] will usually be the same as citations of physical collections, aside from the addition of a URL or DOI.” (CMS16, 14.232)
Guides with Examples of Citations
Chicago Citation Quick Start Guide
Citing Special Collections Materials
How to create Footnotes and Bibliographies.
"Whenever you borrow words or ideas from another source, you need to acknowledge that source. The following situations almost always require a citation:
1. Whenever you use quotes
2. Whenever you paraphrase
3. Whenever you use an idea that someone else has already expressed
4. Whenever you make specific reference to the work of another
5. Whenever someone else's work has been critical in developing your own ideas."
The only time you don't need to cite is when you're using 'common knowledge.'
When do I need to cite? Retrieved June 8, 2012.