# Hypertext Help with LaTeX
\newcommand \renewcommand

`\newcommand{cmd}[args][opt]{def}`
`\renewcommand{cmd}[args][opt]{def}`
`\providecommand{cmd}[args][opt]{def}` -- LaTeX2e

These commands define (or redefine) a command.

- `cmd` The name of the new or redefined command. A `\` followed by a string of lower and/or uppercase letters or a `\` followed by a single nonletter. For `\newcommand` the name must not be already defined and must not begin with `\end`; for `\renewcommand` it must already be defined. The `\providecommand` command is identical to the `\newcommand` command if a command with this name does not exist; if it does already exist, the `\providecommand` does nothing and the old definition remains in effect.

- `args` An integer from 1 to 9 denoting the number of arguments of the command being defined. The default is for the command to have no arguments.

- `opt` (LaTeX2e only) If present, then the first of the number of arguments specified by `args` is optional with a default value of `opt`; if absent, then all of the arguments are required.

- `def` The text to be substituted for every occurrence of cmd; a parameter of the form #n in cmd is replaced by the text of the nth argument when this substitution takes place.

## Examples

`\newcommand{\water}{H$_2$O}`

This would allow one to write, e.g.,

      The formula for water is \water.

or

      \water\ is the formula for water.

Note, in the second case, the trailing `\` followed by a blank is required to ensure a blank space after the H2O; LaTeX ignores the blank following a command, so the space has to be specifically inserted with the `\<space>`.

As a second example consider

`\newcommand{\hypotenuse}{$a^{2}+b^{2}$}`

Note that this will produce the desired formula in text (paragraph) mode because of the `$...$` in the definition. In math mode, however, the first `$` in the definition will cause LaTeX to leave math mode, causing problems.

In LaTeX 2.09 a standard trick for getting around this is to put the math-mode expression in an `\mbox`, viz.,

      \newcommand{\hypotenuse}{\mbox{$a^{2}+b^{2}$}}

In LaTeX2e the `\ensuremath` command has been provided to alleviate this problem. The argument of the `\ensuremath` command is always processed in math mode, regardless of the current mode. Using this mechanism the above could be written as

      \newcommand{\hypotenuse}{\ensuremath{a^{2}+y^{2}}}

See also Commands
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