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The dictionary definition of a footnote1 is something like a note2 usually found at the bottom of the page of a book3 that comments on a certain part of the text4.
Because of their positioning5 readers have to constantly move from where they are reading6 to the bottom of the page, or end of the chapter, and therefore constantly lose their place. This is particularly annoying7 if one is reading such a thing on the internet due to the constant requirement to scroll the page8.
A footnote can also be something done after more important work has been completed9, and thus can be seen as something of an afterthought10.
1Please note that in this article about footnotes no actual dictionary definition has been used. What you will read is merely what that definition may look like if it had been defined as such in a dictionary.2By note, that doesn't mean the text contained within a footnote need be superfluous to the main bulk of the text within the page, although it often is.3Footnotes can also be found at the end of chapters, or in particularly unhelpful books, at the end of the entire script. Those at the end of the books are often called endnotes because they are at the end of the book. Please note that footnotes are so called because they are at the foot of the page, and have nothing at all to do with feet (unless of course the piece of text they are referencing has something to do with feet).4This isn't necessarily true. Footnotes can sometimes have nothing to do with the text at all (see footnote 5).5If you must use the M4, for God's sake don't just sit in the middle lane.6Obviously they don't have to literally get up and move somewhere else. However, had you carried on reading rather than bothering to find this footnote, you would already realise that.7It is generally agreed that this sentence could have ended here and become far more powerful in the process. However, the author decided to continue with something about the Internet.8Alternatively, just ignore the footnotes entirely. This is very difficult though as one assumes there might just be a single special occasion where one misses a gem of information in one of the said footnotes that could utterly change life, the universe, and everything.9This is actually the real reason for footnotes: they allow an author to put things in that they'd previously forgotten without the need to rewrite a large portion of text.10See footnote 9.