The preamble command \underhead places a
rule of length \textwidth a distance of
\underheadsep under the header text. The value
of \underheadsep can be changed in the preamble by
\settolength. This length is added
to the style parameter \headhight, so that the top of the text
is moved down by \underheadsep. To keep the top of the text
in the original position execute the command
\addtolength{\headsep}{-\underheadsep}.
In preparing assignments and classroom notes, I find it convenient to
use the header line as a title, in which case I do not want it
underlined on the first page. Underlining on the current page can be
suppressed by the command \nounderhead.