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

20 Chapter 2. Text, Symbols, and Commands
Note that if the names of the environment in the \begin{..} \end{..}
pair do not match, an error message will be issued on processing.
Most declaration command names (see next section) may also be used
as environment names. In this case the command name is used without
the preceding \ character. For example, the command \em switches to
an emphatic typeface, usually italic, and the corresponding environment
\begin{em} will set all the text in italic until \end{em} is reached.
A nameless environment can be simulated by a {...} pair. The effect
of any command within it ends with the closing curly brace.
The user can even create his or her own environments, as described in
Section 8.4.
2.3 Declarations
A declaration is a command that changes the values or meanings of
certain parameters or commands without printing any text. The effect of
the declaration begins immediately and ends when another declaration
of the same type is encountered. However, if the declaration occurs
within an environment or a {...} pair, its scope extends only to the
corresponding \end command, or to the closing brace }. The commands
\bfseries and \small mentioned in the previous section are examples
of such non-printing declarations that alter the current typeface.
Some declarations have associated arguments, such as the command
\setlength which assigns a value to a length parameter (see Sections 2.4
and 8.2).
Examples:
{\bfseries This text appears in bold face} The \bfseries declaration
changes the typeface: This text appears in bold face. The
effect of this declaration ends with the closing brace }.
\setlength{\parindent}{0.5cm} The paragraph indentation is set to
0.5 cm. The effect of this declaration ends with the next encounter
of the command \setlength{\parindent}, or at the latest with
the \end command that terminates the current environment.
\pagenumbering{roman} The page numbering is to be printed in Roman
numerals.
Some declarations, such as the last example, are global, that is, their
effects are not limited to the current environment. The following declarations
are of this nature, the meanings of which are given later:
\newcounter \pagenumbering \newlength
\setcounter \thispagestyle \newsavebox
\addtocounter