Guide to LaTeX (4th Edition) (Tools and Techniques

184 Chapter 8. User Customizations
8.2 Lengths
It has been constantly pointed out in all the descriptions of the length
parameters such as \parskip or \textwidth that new values may be
assigned with the \setlength command. Some of these parameters
expect rubber length values that may stretch or shrink. These are mainly
parameters that produce vertical spacing. The types of length units,
both fixed and rubber, are described in detail in Section 2.4. This will
not be repeated here, but rather additional commands for assigning and
handling lengths are discussed in this section.
The standard L AT E X method for assigning a value to a length parameter
is with the command
\setlength{\length cmd}{length spec}
where length spec may be a length specification (with units) or another
length parameter. In the latter case, \length cmd takes on the current
value of that other parameter. Thus with \setlength{\rightmargin}
{\leftmargin} the right-hand margin in a list environment is set to
the same value as that of the left-hand margin.
Lengths may be increased with
\addtolength{\length cmd}{length spec}
which adds length spec to the value of the length parameter \length cmd.
A negative value for length spec decreases \length cmd by that amount.
Once again, another length parameter may be used for length spec, with or
without a preceding minus sign, and its value will be added or subtracted.
A decimal number just before a length parameter multiplies its value by
that quantity: 0.5\textwidth means half the width of the text column
and 2\parskip twice the inter-paragraph spacing.
With the command
\settowidth{\length cmd}{text}
the length parameter \length cmd is set equal to the natural length of a
piece of text.
Similarly the commands
\settoheight{\length cmd}{text}
\settodepth{\length cmd}{text}
set the \length cmd equal to the height and depth of the text above and
below the baseline, respectively.
Finally, the command
\stretch{decimal num}
yields a rubber length that is decimal num times as stretchable as \fill
(Section 2.4.2).
A user-defined length parameter is created with