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

!
\newlength{\new len cmd}
8.3. User-defined commands 185
which establishes \new len cmd as a length with a value of 0pt. All the
above commands may be used to manipulate its value further.
The command
\addvspace{length spec}
inserts extra vertical spacing of the amount length spec at that point where it
appears. If more than one such command is given, the total inserted spacing will
be that of the largest argument and not the sum of them all. This command may
only be given between paragraphs. Its application for user-defined commands
and environments lies in the generation of structures that should behave like
paragraphs.
8.3 User-defined commands
New commands may be defined or redefined under L AT E X with the commands
\newcommand{\com name}[narg][opt]{def }
\renewcommand{\com name}[narg][opt]{def }
The first version is used to define a command \com name that does not
yet exist. Its name may be any combination of letters that do not form
the name of another command. The second version redefines an already
existing command \com name. In both cases, an error message is printed
if the incorrect variant is called. The first optional argument narg is a
number between 1 and 9 specifying how many arguments the new or
altered command is to have. A second optional argument opt gives the
default value for an optional argument that the new command may take.
The actual definition of the command is contained in the text def.
8.3.1 Commands without arguments
We will first illustrate the use of the \newcommand without the optional
argument [narg]. This form is applied when a fixed combination of
L AT E X or user commands is to be repeated frequently as a command with
its own name. For example, the structure x1, . . . , xn, called an x-vector,
often occurs in mathematical formulas and is formed in math mode with
x_1,\ldots,x_n. Typing
\newcommand{\xvec}{x_1,\ldots,x_n}
creates a new command named \xvec that may be called and used just
like any other command. When called, it inserts the sequence of text and
commands, in this case x_1,\ldots,x_n, into the current text exactly as