The changes to the preamble commands are intentionally designed to make
LaTeX2e documents look clearly different from old documents. The
commands should be used only before \begin{document}.
\documentclass [<option-list>] {<class-name>}
[<release-date>]
This command replaces the
LaTeX 2.09 command \documentstyle.
There must be exactly one \documentclass command in a document; and
it must come after the filecontents environments, if any, but before
any other commands.
The <option-list> is a list of options, each of which may modify the
formatting of elements which are defined in the <class-name> file,
as well as those in all following \usepackage commands (see
below).
The optional argument <release-date> can be used to specify the earliest desired release date of the class file; it should contain a date in the format YYYY/MM/DD. If a version of the class older than this date is found, a warning is issued.
For example, to specify a two-column article, using a version of
article.cls released after June 1994, you specify:
\documentclass[twocolumn]{article}[1994/06/01]
\documentstyle [<option-list>] {<class-name>}
This command is still supported for compatibility with old files. It
is essentially the same as \documentclass except that it invokes
LaTeX 2.09 compatibility mode. It also causes any options in
the <option-list> that are not processed by the class file to be
loaded as packages after the class has been loaded. See
Section 4 for more details on LaTeX 2.09 compatibility
mode.
\usepackage [<option-list>] {<package-name>} [<release-date>]
Any number of \usepackage commands is allowed. Each package file
(as denoted by <package-name>) defines new elements (or modifies
those defined in the class file loaded by the <class-name> argument
of the \documentclass command). A package file thus extends the
range of documents which can be processed.
The <option-list> argument can contain a list of options, each of which can modify the formatting of elements which are defined in this <package-name> file.
As above, <release-date> can contain the earliest desired release date of the package file in the format YYYY/MM/DD; if an older version of the package is found, a warning is issued.
For example, to load the graphics package for the dvips driver,
using a version of graphics.sty released after June 1994, you write:
\usepackage[dvips]{graphics}[1994/06/01]
Each package is loaded only once. If the same package is requested
more than once, nothing happens in the second or following attempt
unless the package has been requested with options that were not given
in the original \usepackage. If such extra options are specified
then an error message is produced. See Section 6 how
to resolve this problem.
As well as processing the options given in the <option-list> of the
\usepackage command, each package processes the
<option-list> of the \documentclass command as well. This
means that any option which should be processed by every package (to
be precise, by every package that specifies an action for it) can be
specified just once, in the \documentclass command, rather than
being repeated for each package that needs it.
\listfiles
If this command is placed in the preamble then a list of the files read in (as a result of processing the document) will be displayed on the terminal (and in the log file) at the end of the run. Where possible, a short description will also be produced.
Warning: this command will list only files which were read
using LaTeX commands such as \input{<file>} or
\include{<file>}. If the file was read using the primitive TeX
syntax \input file (without { } braces around the file name)
then it will not be listed; failure to use the LaTeX form with the
braces can cause more severe problems, possibly leading to overwriting
important files, so always put in the braces.
\setcounter{errorcontextlines} {<num>}
TeX 3 introduced a new primitive \errorcontextlines which controls
the format of error messages. LaTeX2e provides an interface to this
through the standard \setcounter command. As most LaTeX users do
not want to see the internal definitions of LaTeX commands each time
they make an error, LaTeX2e sets this to -1 by default.