\rule[raise-height]{width}{height}
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.
The \rule
command is fragile.