The footnotes and bibliography in any scholarly work have two purposes:
To do that, your footnotes and bibliography need to include complete and accurate information about your sources, arranged in a consistent way that does not confuse your reader. At this point in your research, you will all have encountered unhelpful footnotes with mysterious abbreviations, incorrect information, or other problems.
There are many ways to arrange the information. This is called "style" and there are several common styles in use. Historians generally prefer the style defined by The Chicago Manual of Style, now in its 16th edition. You may have been asked to use other styles for courses in other departments, for example MLA or APA. Always check with your reader to find out if he/she cares about which style you use. When you write for publication, the publisher or journal editor will tell you which style they want you to use.
Why does it matter? Correct style will make things easier for your reader. And you want the reader to think about your ideas, not the messy punctuation at the bottom of the page.
History department guidelines:
The Chicago Manual of Style. 17th ed. Chicago: University of Chicago Press, 2017.
Slide Show explaining Chicago style citations (below)
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If the Chicago manual seems unapproachable, here are some more concise introductions to Chicago style: