In his great history of the footnote as a literary form, Anthony Grafton argues that the essential innovation of the footnote was to introduce a structure of parallel narrative to historical writing. Once footnotes became the default method of citation in historical scholarship, that is, works of history now came equipped with not one, but two ongoing narratives: first, the author's primary chronological narrative, and second—the sly, often more ironical, discursive editorial narrative that accompanies it through the footnotes.
The reader of a work of history thus has two voices going in their head at the same time, as they work their way through the book: the main historical narrative, plus the editorial commentary.