Synopsis, one of:
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.
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.
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.