You have to use your own judgment on whether a fact needs backing up or not. If you can imagine the reader asking “Says who?” or “How do you know that?” you should add a note. If you write that Abraham Lincoln was an American president, you probably don’t need to cite a source. But if you write that Emperor Gaozu of Tang was the first leader of the Chinese Tang dynasty, you’d better tell where you learned it.
Your source citation of a book, website, article, or dictionary shows that an expert backs up your statement and tells the reader how to find the same facts.
Here’s a one-page tip sheet on when to cite: When to Quote, Paraphrase, or Summarize.
And another one: Three Principles for Citing Sources.
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Students: Many school libraries provide free access to The Chicago Manual of Style Online. If you aren’t sure whether your school subscribes, ask your librarian. In the meantime, click here for a free trial.