Princeton University provides five basic principles for when to cite sources:
- You directly quote a source
- You paraphrase a source
- You summarize someone else's ideas in your own words
- You draw on facts, information, or data from someone else
- You want to add supplemental information not included in your paper, such as footnotes or endnotes
Note: You do not need to cite generally accepted knowledge. For more information, see Not-So-Common Knowledge.
A general rule of thumb is "when in doubt, cite it."