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Whether using electronic or print sources, be careful to attribute ideas to their originators. Imagine, for example, that you find the following quotation on a scholarly blog:
To John Butler Yeats, 14 March 1916. Yeats has been “turning over a book of Japanese paintings” and remarks of their “delight in form” and “ordering of natural objects,” without resort to “imitation.” 18
The author of this blog has done a great deal of work for you. You can’t cite the document simply as “W.B. Yeats, letter to John Butler Yeats, 14 March 1916,” as if you had consulted the original letter. Instead, show the source of your information and indicate that you’re borrowing the editor’s work, as shown in the footnote below. The same is true of a source cited in a print book or any other source.
18 W.B. Yeats, letter to John Butler Yeats, 14 March 1916, quoted in David Ewick, “Japonisme, Orientalism, Modernism: A Bibliography of Japan in English-Language Verse of the Early 20th Century,” 2003. themargins.net/bib/B/BL/bl048.html, accessed November 21, 2015.