To doublespace a LaTeX document, you should include the line
\usepackage{setspace}
after your \documentclass line.
Before your \begin{document} command,
\doublespacing
will make the text of the whole document doublespaced. Footnotes,
figures, and tables will still be singlespaced, however. For one-and-a-half
spacing, instead use the command
\onehalfspacing
In order to make a part of the text of your document singlespaced, you can
put:
\begin{singlespace}
at the beginning of the text you want singlespaced, and
\end{singlespace}
at the end.
You can also set the spacing to be something other than doublespaced; for
example, if you wanted to have one-and-a-quarter spacing between lines,
use the line
\setstretch{1.25}
before your \begin{document} command, and after the
\usepackage{setspace} line.
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