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\footnote

Synopsis, one of:

\footnote{text}
\footnote[number]{text}

Place a footnote text at the bottom of the current page.

There are over a thousand footnotes in Gibbon's 
\textit{Decline and Fall of the Roman Empire}.\footnote{%
  After reading an early version with endnotes David Hume complained, 
  ``One is also plagued with his Notes, according to the present Method 
  of printing the Book'' and suggested that they ``only to be printed 
  at the Margin or the Bottom of the Page.''}

The optional argument number allows you to specify the number of the footnote. If you use this then LaTeX does not increment the footnote counter.

By default, LaTeX uses arabic numbers as footnote markers. Change this with something like \renewcommand{\thefootnote}{\fnsymbol{footnote}}, which uses a sequence of symbols (see \alph \Alph \arabic \roman \Roman \fnsymbol). To make this change global put that in the preamble. If you make the change local then you may want to reset the counter with \setcounter{footnote}{0}.

LaTeX determines the spacing of footnotes with two parameters.

\footnoterule

Produces the rule separating the main text on a page from the page’s footnotes. Default dimensions in the standard document classes (except slides, where it does not appear) is: vertical thickness of 0.4pt, and horizontal size of 0.4\columnwidth long. Change the rule with something like this.

\renewcommand{\footnoterule}{% Kerns avoid vertical space
  \kern -3pt                         % This -3 is negative
  \hrule width \textwidth height 1pt % of the sum of this 1
  \kern 2pt}                         % and this 2

\footnotesep

The height of the strut placed at the beginning of the footnote (see \strut). By default, this is set to the normal strut for \footnotesize fonts (see Font sizes), therefore there is no extra space between footnotes. This is ‘6.65pt’ for ‘10pt’, ‘7.7pt’ for ‘11pt’, and ‘8.4pt’ for ‘12pt’. Change it as with \setlength{\footnotesep}{11pt}.

The \footnote command is fragile (see \protect).

LaTeX’s default puts many restrictions on where you can use a \footnote; for instance, you cannot use it in an argument to a sectioning command such as \chapter (it can only be used in outer paragraph mode; see Modes). There are some workarounds; see following sections.

In a minipage environment the \footnote command uses the mpfootnote counter instead of the footnote counter, so they are numbered independently. They are shown at the bottom of the environment, not at the bottom of the page. And by default they are shown alphabetically. See minipage and Footnotes in a table.