General ⇒ commenting the ends of lines of command arguments

[johnjamessmith0]

I know I've read an explanation about this somewhere, but I couldn't think of a good way to search this question up, sorry.

Will someone explain what is going on when people put '%'s at the ends of lines in multi-line command definitions, and the like, as in

Code: Select all

\def\markboth#1#2{%
  \begingroup
    \let\label\relax \let\index\relax \let\glossary\relax
    \expandafter\@markboth\@themark{#1}{#2}%
    \@temptokena \expandafter{\@themark}%
    \mark{\the\@temptokena}%
  \endgroup
  \if@nobreak\ifvmode\nobreak\fi\fi}

(from fancyhdr's extramarks.sty). I know the practice avoids some problems that tend to occur, but I'd just like to understand why it's necessary so I know when I need it and when I don't.

Thanks.

[meho_r]

When you press "Return/Enter" key and go to another line, a space is automatically added at the end of the previous line in the output. Adding a "%" sign at the end of a line prevents this space from coming into existence (and a space can do much damage when it finds its way into command definitions).

Here's an example, observe the result:

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\documentclass{article}

\begin{document}

Test
Test
Test

Test%
Test%
Test

\end{document}