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Mendeley is a citation management system that allows you to organize citations, store PDFs of articles, cite, and communicate with other scholars. It can be accessed via any web browser and downloaded as a desktop client. The web and desktop versions are meant to be used together.
The BEAM spectrum is a way to think about how you can use sources for your project. Typically historical research uses more of the background and method type of articles. Evidence and argument typically fall under quantitative and qualitative. Use this spectrum to think about how you can use this research to answer your research question.