How Are LaTeX Class Files and Style Files Structured?

A LaTeX class file and a style file can both define commands and load packages, but they have different roles. A class sets the overall document structure and defaults; a package adds a feature to a document that already has a class. This guide shows the usual file layout and a small example you can compile.

The difference between a class and a package

A class file has the extension .cls and is selected in the document preamble with \documentclass. Examples include article, report, and book. A class typically chooses a base class, sets page or heading defaults, and defines class-specific commands.

A style file—usually called a package—has the extension .sty and is loaded with \usepackage. Packages are for reusable features, such as graphics, colors, or commands for a particular subject. A document normally loads one class and can load several packages.

File	Loaded with	Typical purpose
myclass.cls	\documentclass{myclass}	Define a document type and its defaults
mytools.sty	\usepackage{mytools}	Add reusable commands or features

LaTeX finds these files in its search path. While developing, putting them in the same directory as the main .tex file is the simplest approach.

Standard file structure

A basic class file begins like this:

\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{myclass}[2025/03/08 Example document class]
\LoadClass{article}

% Class settings and definitions go here.

\endinput

\NeedsTeXFormat identifies the LaTeX format the file expects. \ProvidesClass declares the class name; it should match the file name, without .cls. \LoadClass loads the base class. A class can then set defaults or define commands and environments. \endinput marks the end of the file for TeX.

A basic package file follows the same pattern but declares itself as a package:

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mytools}[2025/03/08 Example package]
\RequirePackage{ifthen}

% Package settings and definitions go here.

\endinput

The name in \ProvidesPackage should match the file name, without .sty. Use \RequirePackage in a class or package to load dependencies. In a document preamble, use \usepackage instead. Avoid using \documentclass inside a class file or package file: that command belongs in the main document.

A complete working example

This example uses LaTeX’s filecontents* environment to create the two files before the document loads them. In ordinary projects, save each file’s contents separately as myclass.cls, mytools.sty, and demo.tex in the same directory.

\begin{filecontents*}{myclass.cls}
\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{myclass}[2025/03/08 Minimal example class]
\LoadClass[11pt]{article}
\RequirePackage{mytools}
\newcommand{\classgreeting}{This command is defined by the class.}
\endinput
\end{filecontents*}

\begin{filecontents*}{mytools.sty}
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mytools}[2025/03/08 Minimal example package]
\newcommand{\toolgreeting}{This command is defined by the package.}
\endinput
\end{filecontents*}

\documentclass{myclass}
\title{A Small Example}
\author{A. Writer}

\begin{document}
\maketitle

\classgreeting\par
\toolgreeting
\end{document}

The class loads article as its base and then requires the package. The main document only needs to select myclass; it can use commands provided by both files. Compile the example with a current LaTeX distribution. For separate files, the class and package must be saved before compiling demo.tex.

Class options and package options

A class can accept options and pass them to its base class. For example, this forwards any otherwise-unrecognized class option to article:

\DeclareOption*{\PassOptionsToClass{\CurrentOption}{article}}
\ProcessOptions\relax
\LoadClass{article}

Put the option declarations and \ProcessOptions before \LoadClass. Then a document using \documentclass[12pt]{myclass} passes 12pt to article. Packages can also declare and process options; use \DeclareOption, \ProcessOptions\relax, and, when needed, \PassOptionsToPackage.

For a first version, keep option handling out unless the file actually needs configurable behavior. Add it deliberately and check that every accepted option has the intended effect.

Common mistakes

• Mismatched names: \ProvidesClass{myclass} belongs in myclass.cls; \ProvidesPackage{mytools} belongs in mytools.sty.
• Using the wrong loading command: select a class with \documentclass, but load a package with \usepackage or \RequirePackage from another class or package.
• Loading a base class from a package: \LoadClass is for a class file. A package should extend the document’s existing class instead.
• Using \input for dependencies: \RequirePackage is the standard package-loading command. It provides package-aware loading and avoids accidentally loading a package more than once.
• Editing a system class directly: copy or create a custom class with a distinct name rather than modifying a TeX distribution’s built-in files, which updates may replace.
• Leaving definitions untested: compile a tiny document that loads the file and exercises its commands. This makes missing dependencies and naming errors easier to locate.

As a rule, put document-wide structure and defaults in a .cls file, and reusable, independently loadable functionality in a .sty file. A class may require packages, and a package may define many commands, but each file should have one clear role.