How do I cite a source that's quoting a second source?

It sounds complicated, but it's not!

Different citation styles have slightly different advice for these instances, but I'm going to base my comments on the Chicago Manual of Style, as read by Kate Turabian for the notes-bibliography style. It's the most common at Iliff.

This is what Turabian calls "One Source Quoted in Another" (17.10). Her first advice is to actually go to the original source of the quotation to make sure that your second source is using the material in a fair and accurate way. However, if the original source is unavailable to check, you'll need to cite it as "quoted in"-- what follows are the examples from Turabian:

In notes:

8. Louis Zukofsky, "Sincerity and Objectification," Poetry 37 (February 1931): 269, quoted in Bonnie Costello, Marianne Moore: Imaginary Possessions (Cambridge, MA: Harvard University Press, 1981), 78.

And in your bibliography:

Zukofsky, Louis. "Sincerity and Objectification." Poetry 37 (February 1931): 269. Quoted in Bonnie Costello, Marianne Moore: Imaginary Possessions. Cambridge, MA: Harvard University Press, 1981.