How to underline section-headings in LaTeX?

Question

EDIT: As I see in the first answer, underline is the wrong word. I want a line under the section heading, separating the heading from the following text.

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Answer 1

You might take a look at the LaTeX \rule command:

\rule[raise-height]{width}{thickness}

However, that's an inline (horizontal) command, so you'll have to use some trickery to get it at the right position and size (\noindent and \textwidth should help a little).

Otherwise, you can use the following plain TeX

\smallskip
\hrule height 2pt
\smallskip

It doesn't really fit in the LaTeX syntax, but it works like a charm, and is guaranteed to span the entire text width.