Source: Ohio State University Heterick Memorial Library https://libguides.onu.edu/c.php?g=357773&p=2720217
Some items can be both primary and secondary sources depending on how you use them in your research. If you are studying the Civil War, then scholarly articles written in the 20th and 21st Century would be considered secondary sources. However, if you are studying the history of Civil War scholarship, then those same items would be considered primary resources.
When in doubt, ask a libriarian!