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Trying to answer to Include a cover page then title page in book class I've found that \newpage only creates a new page if there is some contents, i.e.,

\documentclass{article}
\begin{document}
\newpage
\end{document}

doesn't produce any result (no pdf file) while

\documentclass{article}
\begin{document}
\null\newpage
\end{document}

does it (empty page with 1 on footer). Instead of \null, \mbox{} also works. Could you explain why?

My understanding is that an empty page is not empty because LaTeX automatically introduces headers and footers. I was using \pagestyle{empty} or \thispagestyle{empty} could understand it, but first code doesn't fit this case.

share|improve this question
1  
Very informal idea: Well, to create a new page \newpage you need at least one page, otherwise you can not go to the next one. So, a page must contain at least an empty char. – Sigur Jan 26 at 18:08
up vote 16 down vote accepted

Headers and footers are added after TeX has decided it wants to do a page break, so the fact that they are empty or not is completely irrelevant.

A page must have something non discardable in it (a box, a rule, a whatsit), because discardable items (glue, penalties, kerns) are discarded at a page break.

So doing \newpage\newpage does not produce a blank page because, when issued in vertical mode, it essentially does

\vfil
\penalty-10000

Note that the start of a document is implicitly considered as a page break, so initial discardable items are indeed discarded. That's the same reason why you get no vertical space when you do

\begin{document}
\vspace{10cm}
Hello world
\end{document}

and “Hello world” will appear at the top.

The macro \null does \hbox{}, so it provides a box. A better method for adding something invisible is doing

\vspace*{0pt}

There's a very good reason for this: after the final \clearpage (that implicitly calls \newpage) issued by \end{document}, TeX is still doing its work and it's ready to make a new page, because pending floats might remain. If its behavior was changed to allow \newpage making a page without non discardable items, you'd always get a blank page at the end.


This is the full definition of \newpage:

% latex.ltx, line 6503 (2015/10/01 patch level 2)
\def \newpage {%
  \if@noskipsec
    \ifx \@nodocument\relax
      \leavevmode
      \global \@noskipsecfalse
    \fi
  \fi
  \if@inlabel
    \leavevmode
    \global \@inlabelfalse
  \fi
  \if@nobreak \@nobreakfalse \everypar{}\fi
  \par
  \vfil
  \penalty -\@M}

This opens the problem of where \if@noskipsec is true; well, it is at startup, but it is set to false at \begin{document} and back to true when a run-in section title is being typeset. The command \@nodocument is responsible of issuing an error message if some text is typeset before \begin{document}, but is later set to be equivalent to \relax. So the purpose of the first conditional is to put things into order when something like \paragraph{Title}\newpage is found.

The conditional \if@inlabel is true when LaTeX is processing an \item, so that part is for recovering from input such as \item\newpage. Finally \if@nobreak is true when LaTeX has typeset a “block” sectional title (\section, for instance) and, again, this is for recovering from something like \section{Title}\newpage.

So the initial part is just code that tries to avoid later problems in presence of disputable code. If none of those (hopefully) rare situations occurs, then just \par\vfil\penalty-10000 is issued. Note that \par is normally implicit in \vfil, but just in case the user has redefined \vfil, it is issued nonetheless.

I don't recommend \leavevmode\newpage that still appears to work; the command does start a paragraph, contributing \parskip glue and \baselineskip glue too. To the contrary, \null or \vspace*{0pt} just contribute a box/rule of zero height and depth (the latter also a 10000 penalty and zero glue), with no \parskip and \baselineskip glue.

Not a big deal, actually, since we're analyzing the problem of making an apparently empty page: a blank box, an invisible rule or a blank paragraph don't make such a big difference, but the less we contribute the better. Starting a paragraph also frees the contents of \everypar, which can have some side effect in some situations.

share|improve this answer
1  
+1. Could you explain why \vspace*{0pt} is better? – Sigur Jan 26 at 18:36
2  
@Sigur Because it's documented in the manual and \null isn't. – egreg Jan 26 at 18:36
    
I'd have issued \leavevmode\newpage. – A.Ellett Jan 26 at 19:16
    
@A.Ellett \leavevmode isn't a documented latex command either and the behaviour of \leavevmode\newpage is much more complicated to analyse (and less desirable) than either null or vspace* (I'll leave that to egreg:-) – David Carlisle Jan 26 at 20:23
1  
@DavidCarlisle So that opens a question for me: what does "documented LaTeX command" mean? Since, after all, I can find \leavevmode in source2e but I can't find \vspace*{0pt} in source2e. Where should I be looking for such documentation? – A.Ellett Jan 26 at 20:27

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