A footnote is a note referenced from the text and placed at the bottom of a book or scholarly journal, or, more rarely, beside the referencing text in a book or document.
A footnote usually contain a reference, a citation, or comments that the author decided not to include within the main body of the text because it would interrupt the flow of the argument. The footnote is usually referenced in the main text by a superscripted letter, symbol or an ordinal number, but other forms and formats are also common. A special type of note are the endnotes which are similar in most respects to footnotes, but are positioned at the end of a chapter or a book.
Footnotes originated in the 16th century and became common in scholarly books in the Enlightenment (late 18th century), and were adopted by most of the academic journals that appeared in the late 19th century. The purpose is to validate the scholarship by citing the author's sources. They allow the reader to see how thoroughly familiar the author is with the literature specific to a discipline; to read the original source in its context in order to compare the author's interpretation of the source with the reader's own; or, in the case of foreign-language sources, to check the author's translation. The footnotes allow the author to debate specialized points with other specialists . Publishers find that most readers, not familiar with specialized literature, dislike footnotes and so publishers hide them at the end of a chapter or end of a book. In rare cases major academic books (such as those by Perry Miller) are published without footnotes, or with the footnotes on the publisher's website but not in the printed book itself (as Yale University Press did in 2001 with Michael Nylan's The Five "Confucian" Classics).