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\rule

Synopsis, one of:

\rule{width}{thickness}
\rule[raise]{width}{thickness}

Produce a rule, a filled-in rectangle.

This produces a rectangular blob, sometimes called a Halmos symbol, often used to mark the end of a proof.

\newcommand{\qedsymbol}{\rule{0.4em}{2ex}}

The amsthm package includes this command, with a somewhat different-looking symbol.

The mandatory arguments give the horizontal width and vertical thickness of the rectangle. They are rigid lengths (see Lengths). The optional argument raise is also a rigid length, and tells LaTeX how much to raise the rule above the baseline, or lower it if the length is negative.

This produces a line, a rectangle that is wide but not tall.

\noindent\rule{\textwidth}{0.4pt}

The line is the width of the page and 0.4 points tall. This line thickness is common in LaTeX.

A rule that has zero width, or zero thickness, will not show up in the output, but can cause LaTeX to change the output around it. See \strut, for examples.