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.
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.