Help On LaTeX fragile commands

# Hypertext Help with LaTeX
Fragile and Robust commands

LaTeX commands are either robust or fragile. Fragile commands need special care if they are part of a moving argument.

Arguments to some LaTeX commands are called moving arguments because they may be "moved" to other places in the document. For example, the argument of Sectioning comands may be moved to the Table of Contents. Other examples of moving arguments include:

- arguments of `\caption` commands
- terminal input and output, `\typeout` and `\typein`
- commands that produce page headings
- the Letter Environment
- the `\thanks` command
- an `@` expression in the Array or Tabular environment

A fragile command that appears in a moving argument must be preceded by a `\protect` command. The `\protect` applies only to the immediately following command; if arguments of this command also contain fragile commands, the latter must be protected with their own `\protect`.

## Some fragile commands

The following list is not exhaustive, but illustrates the kind of commands which are fragile

- All commands that have an optional argument are fragile.
- Environments delimited by `\begin ... \end` are fragile.
- Display math environment delimited by `\[ ... \]`
- Math environment `\( ... \)`
  - However, `$ ... $` is robust
- Line breaks, `\\`
- `\item` commands
- `\footnote` commands

## Some robust commands

In general, commands which change Type face or Type style are robust.

Length commands are robust and should not be preceded by a `\protect` command. Nor should a `\protect` command be used in the argument to `\addtocounter` or `\setcounter` command.

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