With the following MWE, i get the Section title underlined in black :


{Section \arabic{section}}%

\section{mon titre}

My first problem : if i remove the { } enclosing \titlerule, i get a compilation error

! LaTeX Error: Missing \begin{document}

Which is strange, since the content of the [ ] is a text to typeset after the section title. For example


works. My second problem : if i colorize the rule with


I obtain a blue underline, but with an additional vertical space (app 0.5 cm) between the title and the underline. If i use


instead, the vertical space disappears... I have to admit, i do not understand that.

  • To see why \textcolor starts a new paragraph (mentioned in @egreg's answer), add \makeatletter\show\@textcolor\makeatother after \titleformat, then inspect your .log file and see that it executes \leavevmode (\@textcolor is just a subsidiary function called by \textcolor). Then read Function and usage of \leavevmode. – Werner Jan 4 '12 at 22:01

It's not strange: without the braces, in the first case, the first closing ] is matched with the opening [ that announces the optional argument to \titleformat. An optional argument always goes from the [ to the first ] (at the same brace level).

The second problem is due to the fact that \textcolor starts a paragraph, while \color doesn't. But, as you already discovered, using \color inserts a break point and the rule may go on the following page.


  {Section \arabic{section}}
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