A \gls*{command} is used to tell \LaTeX\ to do a particular thing
at that point in the document. These are the basic forms a command
can take:

\begin{enumerate}
\item \label{itm:controlword}\textbf{A Control Word.}\index{command!control word|hyperpage}

This is a backslash \gls{backslashchar} followed by letters (A,\ldots,Z,a,\ldots,z). \faq{Commands gobble following space}{xspace}There can be no
non-alphabetical characters in the command, apart from the initial
backslash, and the name is always \textbf{case-sensitive} so, for example, \glsni{gamma}
and \glsni{Gamma} have different meanings. One command
that often trips up new users is \gls{LaTeX}, which prints the
LaTeX logo: \makeimg{LaTeX logo}{\LaTeX}. This command has three
captial letters and two lower case letters. If you get the case
of any of the letters incorrect, you will get an \dq{undefined
control sequence} error.

\strut\warning There must be no space between the backslash and the
start of the command name. Some command names are made up of two
or more names joined together, such as \glsni{tableofcontents}.
\emph{Make sure you don't insert any spaces in the control word.} This will
either lead to an error or an unexpected result.
For example, 
\begin{alltt}\correct
\cmdname{appendixname}
\end{alltt}
displays \dq{\appendixname} but 
\begin{alltt}\wrong
\cmdname{appendix name}
\end{alltt}
switches to the appendices and then prints the word \dq{name}.

Most \LaTeX\ commands have fairly self-explanatory names. 
(For example, \booklinebreak\glsni{chapter} starts a new chapter and 
\glsni{rightarrow} prints an arrow pointing to the right.)
However, in most cases, you need to use U.S. spelling
(for example, \glsni{color} rather than \cmdname{colour}).

This is the most common form of command. Any spaces immediately
following a command of this type are ignored, so for example
\begin{codeS}
\gls{TeX} nician
\end{codeS}%
  will produce
\begin{resultS}[TeXnician (where the TeX logo is written with a dropped E)]
\TeX nician
\end{resultS}%
  whereas
\begin{codeS}
\glsni{TeX}\marg{} nician
\end{codeS}%
  will produce
\begin{resultS}[TeX nician (where the TeX logo is written with a dropped E)]
\TeX{} nician
\end{resultS}%
\bookpagebreak
  But the following will cause an \dq{undefined control sequence}
  error:
\begin{alltt}\wrong
\cmdname{TeXnician}
\end{alltt}

There is one command that you must use in every document you create,
and that is the \glsi{documentclass} command.  This command
must be placed at the very start of your document, and indicates what
type of document you are creating.  This command is described in more detail
\latex{in }\chapterref[later]{ch:simpledoc}.

\item \label{itm:starredcommand}\textbf{A Starred Command}\index{command!starred|hyperpage}

Some commands have variants that are indicated by an asterisk at the
end of the name. For example, \verb|\chapter| makes a numbered
chapter whereas \verb|\chapter*| is makes an unnumbered chapter.
A \keyword{starred command} is the version of the command with the
asterisk. (On a UK keyboard the asterisk character is usually
located on the same key as the digit~\texttt{8}.)

This may seem like a different form to a control word, described
above. After all, I've just said that a control word can only
contain alphabetical characters. However a starred command is actually a control
word (such as \glsi{chapter}) followed by an asterisk. The
control word checks to see if the next character is
an asterisk. If it is, it performs one action, otherwise it performs
another action.

This type should therefore just come under the previous category,
but as you will often hear of \dq{starred commands} it seemed better
to have a separate category. 

\item \label{itm:controlsymbol}\textbf{A Control Symbol.}\index{command!control
symbol|hyperpage}

This is a backslash followed by a single non-alphabetical character.
For example \glsi{percent} will print a percent symbol.
Spaces are
not ignored after this type of command, for example
\begin{codeS}
17.5\glsi{percent} VAT
\end{codeS}%
will produce
\begin{resultS}[vat.html]
17.5\% VAT
\end{resultS}

It's also possible to have starred forms of control symbols. For
example \glsi{newline.dbbackslashchar} forces a line break. If it's
not followed by an asterisk a page break is allowed at that line
break, but if it is followed by an asterisk \verb|\\*| no page break is allowed
at that line break. (If a page break is needed, it will be made at
the end of the previous line instead.)

\item \label{itm:charactersequence}\textbf{Character Sequence.}\index{command!character
sequence|hyperpage}

Some special sequences of characters combine to form an instruction. For example \texttt{ffi}
is the command to produce the \makeimg{ffi}{ffi} ligature, and the sequence
of symbols \iexclamdowncmd\ is the command to produce the upside
down exclamation mark !`

\item \textbf{An Internal Command.}\index{command!internal|hyperpage}

This is like the first type, a control word, but the
\texttt{@}\index{"@ in a command name@\texttt{"@} (in a command name)|hyperpage}
character appears in the command name (for example
\cmdname{c@section}) \emph{however} internal commands should only be
used in \htmlref{class files}{sec:cls} or
\htmlref{packages}{sec:packages}. The @ symbol takes on a special
meaning when a file is included using \glsi{documentclass} (a
class file) or \glsi{usepackage} (a package).

For example, in a class file or package \cmdname{c@section} is an
internal representation of the section
\htmlref{counter}{ch:counters}, whereas in a \texttt{.tex} file
\cmdname{c@section} is interpreted as the command \cmdname{c} (the
cedilla \htmlref{accent command}{obj:accents}) that takes the
character @ as its argument, followed by \texttt{section}, which
produces the rather odd looking 
\makeimg{@ with a cedilla section}{\c{@}section}\faq{\textbackslash @ and @ in macro
names}{atsigns}.

Don't be tempted to use internal commands until you have first
grasped the basics. You have been warned!

\end{enumerate}