The \rule command generates a rectangular "blob of ink."
It can be used to produce horizontal or vertical lines. The arguments
are defined as follows.
raise-height specifies how high to raise the rule (optional)
width specifies the length of the rule (mandatory)
height specifies the height of the rule (mandatory)
The default value for raise-height is zero; a negative value
lowers the rule.
The reference point of the rule box is the lower left-hand corner.
A rule box of zero width may be used as a "strut" which is invisible but
which causes LaTeX to adjust veritcal spacing to leave room. This can
be particularly useful where the
\vspace command cannot be used,
for example, within mathematical formulas.