How can I write the character _ in an \fbox?
\fbox{PART one_Part two}
Doesn't work.
How can I write the character _ in an \fbox?
\fbox{PART one_Part two}
Doesn't work.
Use \_ or \textunderscore. The problem is that, on its own, _ is a reserved LaTeX character like % and that is why you are getting the error. The fact that it is inside a fbox should not be the issue. For a complete list of reserved characters see http://en.wikibooks.org/wiki/LaTeX/Basics#Reserved_Characters.