When TeX is building a paragraph, it can make several attempts to get the line-breaking right; on each attempt it runs the same algorithm, but gives it different parameters. You can affect the way TeX’s line breaking works by adjusting the parameters: this answer deals with the “tolerance” and stretchability parameters. The other vital ‘parameter’ is the set of hyphenations to be applied: see “my words aren’t being hyphenated” (and the questions it references) for advice.
If you’re getting an undesired “overfull box”, what has happened is that TeX has given up: the parameters you gave it don’t allow it to produce a result that doesn’t overfill. In this circumstance, Knuth decided the best thing to do was to produce a warning, and to allow the user to solve the problem. (The alternative, silently to go beyond the envelope of “good taste” defined for this run of TeX, would be distasteful to any discerning typographer.) The user can almost always address the problem by rewriting the text that’s provoking the problem — but that’s not always possible, and in some cases it’s impossible to solve the problem without adjusting the parameters. This answer discusses the approaches one might take to resolution of the problem, on the assumption that you’ve got the hyphenation correct.
The simplest case is where a ‘small’ word fails to break at the end of
a line; pushing the entire word to a new line isn’t going to make much
difference, but it might make things just bad enough that TeX won’t
do it by default. In such a case on can try the LaTeX
\linebreak command: it may solve the problem, and if it does, it
will save an awful lot of fiddling. Otherwise, one needs to adjust
parameters: to do that we need to recap the details of TeX’s line
breaking mechanisms.
TeX’s first attempt at breaking lines is performed without even
trying hyphenation: TeX sets its “tolerance” of line breaking
oddities to the internal value \pretolerance, and sees what
happens. If it can’t get an acceptable break, TeX adds the
hyphenation points allowed by the current patterns, and tries again
using the internal \tolerance value. If this pass also fails, and
the internal \emergencystretch value is positive, TeX will try
a pass that allows \emergencystretch worth of extra stretchability
to the spaces in each line.
In principle, therefore, there are three parameters (other than
hyphenation) that you can change: \pretolerance, \tolerance
and \emergencystretch. Both the tolerance values are
simple numbers, and should be set by TeX primitive count
assignment — for example
For both, an “infinite” tolerance is represented by the value 10000, but infinite tolerance is rarely appropriate, since it can lead to very bad line breaks indeed.\pretolerance=150
\emergencystretch is a TeX-internal ‘dimen’ register, and can
be set as normal for dimens in Plain TeX; in LaTeX, use
\setlength — for example:
\setlength{\emergencystretch}{3em}
The choice of method has time implications — each of the
passes takes time, so adding a pass (by changing
\emergencystretch) is less desirable than suppressing one (by
changing \pretolerance). However, it’s unusual nowadays to find a
computer that’s slow enough that the extra passes are really
troublesome.
In practice, \pretolerance is rarely used other than to manipulate
the use of hyphenation; Plain TeX and LaTeX both set its value
to 100. To suppress the first scan of paragraphs, set
\pretolerance to -1.
\tolerance is often a good method for adjusting spacing;
Plain TeX and LaTeX both set its value to 200. LaTeX’s
\sloppy command sets it to 9999, as does the
sloppypar environment. This value is the largest
available, this side of infinity, and can allow pretty poor-looking
breaks (this author rarely uses \sloppy “bare”, though he does
occasionally use sloppypar — that way, the change of
\tolerance is confined to the environment). More satisfactory is
to make small changes to \tolerance, incrementally, and then to look to
see how the change affects the result; very small increases can often
do what’s necessary. Remember that \tolerance is a paragraph
parameter, so you need to ensure it’s actually applied — see
“ignoring paragraph parameters”.
LaTeX users could use an environment like:
\newenvironment{tolerant}[1]{%
\par\tolerance=#1\relax
}{%
\par
}
enclosing entire paragraphs (or set of paragraphs) in it.
The value of \emergencystretch is added to the assumed
stretchability of each line of a paragraph, in a further run of the
paragraph formatter in case that the paragraph can’t be made to look
right any other way. (The extra scan happens if
\emergencystretch>0pt — if it’s zero or negative, no gain
could be had from rerunning the paragraph setter.) The example above
set it to 3em; the Computer Modern fonts ordinarily fit three
space skips to the em, so the change would allow anything up
to the equivalent of nine extra spaces in each line. In a line with
lots of spaces, this could be reasonable, but with (say) only three
spaces on the line, each could stretch to four times its natural
width. It is therefore clear that \emergencystretch needs to be
treated with a degree of caution.
More subtle (but more tricky to manage) are the microtypographic
extensions provided by PDFTeX. Since PDFTeX is the default
‘engine’ for LaTeX and CONTeXT work in all distributions,
nowadays, the extensions are available to all. There are two
extensions, margin kerning and font expansion; margin kerning only
affects the visual effect of the typeset page, and has little effect
on the ability of the paragraph setter to “get things right”.
Font expansion works like a subtler version of the trick that
\emergencystretch plays: PDFTeX ‘knows’ that your current font
may be stretched (or shrunk) to a certain extent, and will do that
“on the fly” to optimise the setting of a paragraph. This is a
powerful tool in the armoury of the typesetter.
As mentioned above, the microtypographic extensions are tricky beasts to control; however, the microtype package relieves the user of the tedious work of specifying how to perform margin adjustments and how much to scale each font … for the fonts the package knows about; it’s a good tool, and users who can take on the specification of adjustments for yet more fonts are always welcome.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=overfull