A \gls{length} register stores dimensions (such as 1in, 5cm,
8.25mm). Like \glslink{command}{control words}, length registers
start with a backslash and only contain alphabetical characters in
their name. These registers are used to determine page layouts etc.
For example, the paragraph indentation is given by the length
register \gls{parindent}. Acceptable units of measurement are listed
in \tableref{tab:units}. The two relative units \dq{em} and \dq{ex} are 
dependent on the current font. (The em-value used to be the
width of an \dq{M} and the ex-value was the height of the letter
\dq{x}, but these days the values are more
arbitrary~\protect\cite{texbook}.) Use \texttt{em} for widths and
\texttt{ex} for heights if you want to use relative values.

\begin{table}[htbp]
\caption{Units of Measurement}
\label{tab:units}
\centering
\begin{tabular}{ll}
\iunit{pt}{\protect\TeX\ point} & \TeX\ point: 72.27pt = 1in\\
\iunit{in}{inch} & inch: 1in = 25.4mm\\
\iunit{mm}{millimetre} & millimetre: 1mm=2.845pt\\
\iunit{cm}{centimetre} & centimetre: 1cm = 10mm\\
\iunit{ex}{relative unit} & the \dq{x-height} of the current font\\
\iunit{em}{relative unit} & the width of a \dq{quad} in the current font\\
\iunit{sp}{scaled point} & scaled point: 1sp = 65536pt\\
\iunit{bp}{big point} & big point (or \Index{PostScript} point): 72bp = 1in\\
\iunit{dd}{did\^ot point} & did\^ot point: 1dd=0.376mm\\
\iunit{pc}{pica} & pica: 1pc=12pt\\
\iunit{cc}{cicero} & cicero: 1cc=12dd\\
\iunit{mu}{math unit} & math unit: 18mu = 1em
\end{tabular}
\par
\end{table}

To change a length you can use the command:
\begin{definition}
\gls{setlength}\marg{\meta{cmd}}\marg{\meta{length}}
\end{definition}%
where \meta{cmd} is the register (for example, \glsni{parindent})
and \meta{length} is the new length. Alternatively, you can add a
value to a length using:
\begin{definition}
\gls{addtolength}\marg{\meta{cmd}}\marg{\meta{length increment}}
\end{definition}
The value of a length register can be displayed in your document using
\begin{definition}
\gls{the}\meta{register}
\end{definition}

A \keyword{rubber length} is a length that has a certain amount of
elasticity. This enables you to specify your desired length but
allows \LaTeX\ to stretch or contract the space to get the body of
text as flushed with the margins as possible.

\faq{Zero paragraph indent}{parskip}For example, the paragraph gap \gls{parskip} is usually set to
\texttt{0pt plus 1pt}.  This means that the preferred gap is
0pt but \LaTeX\ can stretch it up to 1pt to help prevent the
page from having a ragged bottom. For example:
\begin{code}
\begin{alltt}
\glsni{setlength}\marg{\glsni{parindent}}\marg{0pt}
\glsni{setlength}\marg{\glsni{parskip}}\marg{10pt plus 1pt minus 1pt}

This is the first paragraph.

This is the second paragraph.
The paragraph indentation is \glsni{the}\glsni{parindent}.
\end{alltt}
\end{code}%
This now produces:
\begin{result}
\setlength{\parindent}{0pt}
\setlength{\parskip}{10pt plus 1pt minus 1pt}

This is the first paragraph.

This is the second paragraph.
The paragraph indentation is \the\parindent.
\end{result}%
In this example, the preferred paragraph gap is 10pt but
it will allow for a deviation of up to plus or minus 1pt.

\warning Note that it's generally best not to change \glsni{parskip}
explicitly as it can cause unexpected complications. If you use one
of the \koma\ classes, such as \icls{scrreprt}, you can use the
\scrclsopt{parskip} class option that can take the following values:
\texttt{parskip=full} (a full line height) \texttt{parskip=half}
(half a line height).\footnote{There are also variants that have
\texttt{+}, \texttt{-} or \texttt{*} as a suffix. See the 
\koma\ documentation for further details.}

\xminisec{Example:}
\begin{codeS}
\glsni{documentclass}\oarg{parskip=full}\marg{scrbook}
\end{codeS}

If you want to change any of the page layout lengths (such as
\gls{textwidth}), the easiest way to do it is to use the
\isty{geometry} \htmlref{package}{sec:packages}.  This package should have been installed
when you installed your \TeX\ distribution. For example: suppose you want the total text
area to be 6.5in wide and 8.75in high with a left margin of 0.4in, then you would do:
\begin{codeS}
\glsni{usepackage}\oarg{body=\marg{6.5in,8.75in}\comma left=0.4in}\marg{geometry}
\end{codeS}