Preventing page breaks between lines

One commonly requires that a block of typeset material be kept on the same page; it turns out to be surprisingly tricky to arrange this.

LaTeX provides a samepage environment which claims it does this sort of thing for you. It proceeds by setting infinite penalties for all sorts of page break situations; but in many situations where you want to prevent a page break, samepage doesn’t help. If you’re trying to keep running text together, you need to end the paragraph inside the environment (see preserving paragraph parameters). Also, if the things you are trying to keep together insert their own page break hints, samepage has no power over them (though list items’ attempts — they suggest page breaks between items — are subverted by samepage). Naturally, if samepage does work, it is capable of leaving stuff jutting out at the bottom of the page.

Another convenient trick is to set all the relevant stuff in a \parbox (or a minipage if it contains things like verbatim text that may not be used in the argument of a \parbox). The resulting box certainly won’t break between pages, but that’s not to say that it will actually do what you want it to do: again, the box may be left jutting out at the bottom of the page.

Why do neither of these obvious things work? — Because TeX can’t really distinguish between infinitely awful things. Samepage will make any possible break point “infinitely bad” and boxes don’t even offer the option of breaks, but if the alternative is the leave an infinitely bad few centimetres of blank paper at the bottom of the page, TeX will take the line of least resistance and do nothing.

This problem still arises even if you have \raggedbottom in effect: TeX doesn’t notice the value of that until it starts actually shipping a page out. One approach is to set:

\raggedbottom
\addtolength{\topskip}{0pt plus 10pt}

The 10pt offers a hint to the output routine that the column is stretchable; this will cause TeX to be more tolerant of the need to stretch while building the page. If you’re doing this as a temporary measure, cancel the change to \topskip by:

\addtolength{\topskip}{0pt plus-10pt}

as well as resetting \flushbottom. (Note that the 10pt never actually shows up, because it is overwhelmed when the page is shipped out by the stretchability introduced by \raggedbottom; however, it could well have an effect if \flushbottom was in effect.)

An alternative (which derives from a suggestion by Knuth in the TeXbook) is the package needspace or the memoir class, which both define a command \needspace whose argument tells it what space is needed. If the space isn’t available, the current page is cleared, and the matter that needs to be kept together will be inserted on the new page. For example, if 4 lines of text need to be kept together, the sequence

\par
\needspace{4\baselineskip}
% the stuff that must stay together
<text generating lines 1-4>
% now stuff we don't mind about

Yet another trick by Knuth is useful if you have a sequence of small blocks of text that need, individually, to be kept on their own page. Insert the command \filbreak before each small block, and the effect is achieved. The technique can be used in the case of sequences of LaTeX-style sections, by incorporating \filbreak into the definition of a command (as in patching commands). A simple and effective patch would be:

\let\oldsubsubsection=\subsubsection
\renewcommand{\subsubsection}{%
  \filbreak
  \oldsubsubsection
}

While the trick works for consecutive sequences of blocks, it’s slightly tricky to get out of such sequences unless the sequence is interrupted by a forced page break (such as \clearpage, which may be introduced by a \chapter command, or the end of the document). If the sequence is not interrupted, the last block is likely to be forced onto a new page, regardless of whether it actually needs it.

If one is willing to accept that not everything can be accomplished totally automatically, the way to go is to typeset the document and to check for things that have the potential to give trouble. In such a scenario (which has Knuth’s authority behind it, if one is to believe the rather few words he says on the subject in the TeXbook) one can decide, case by case, how to deal with problems at the last proof-reading stage. At this stage, you can manually alter page breaking, using either \pagebreak or \clearpage, or you can place a \nopagebreak command to suppress unfortunate breaks. Otherwise, you can make small adjustments to the page geometry, using \enlargethispage. Supposing you have a line or two that stray: issue the command \enlargethispage{2\baselineskip} and two lines are added to the page you’re typesetting. Whether this looks impossibly awful or entirely acceptable depends on the document context, but the command remains a useful item in the armoury.

Note that both \pagebreak and \nopagebreak take an optional number argument to adjust how the command is to be interpreted. Thus \pagebreak[0], the command “suggests” that a page break might be worth doing, whereas \pagebreak[4] “demands” a page break. Similarly \nopagebreak[0] makes a suggestion, while \nopagebreak[4] is a demand. In both commands, the default value of the optional argument is 4.

FAQ ID: Q-nopagebrk
Tags: layout