[texhax] hline or underline in align environment

[John Wheeler]

I know this must be well-known, but I couldn't find it in the faq with my
limited knowledge of the searching.

I'm using the align environment (having gratefully graduated from eqnarray
at the suggestion of this group) and need to be able to put an underline
beneath one equation before proceeding to the next, as in adding two equations
to obtain a third.  The line should extend just under the width of the
equations, not across the entire page, nor starting from the left-hand side
of the page.  When I use \\\hline at the end of one equation, I get a line
coming in from the left-hand side of the page.  (The same problem occurs with
eqnarray, but then the line crosses the entire page).  Underline doesn't work
because each piece of the equation must be underlined separately, and the 
underlines don't line up.  Surely there must be a command that does this 
painlessly?  (I finally fell back on tabular{l c r} in which \hline works as
desired, but then the spacing is less than perfect.)
Thanks, and please keep the flames for not knowing where to look gentle.
John Wheeler
jcw at chemj2.ucsd.edu