Document Structure - Foundations Course

Every LaTeX document follows the same skeleton: a document class declaration, a preamble where you load packages and define settings, and a body where your content lives. Understanding this structure is the foundation for everything else in this book.

The Anatomy of a `.tex` File

The three regions of every LaTeX document.

\documentclass[11pt,a4paper]{article}   % 1. Class declaration

% === PREAMBLE ===                       % 2. Preamble
\usepackage{amsmath}
\usepackage{graphicx}
\title{My Document}
\author{Your Name}
\date{\today}

\begin{document}                         % 3. Body begins

\maketitle
\tableofcontents

\section{Introduction}
Your content goes here.

\end{document}                           % 3. Body ends

The Document Class

The \documentclass command is always the first line. It sets the overall document type:

Class: article | Use Case: Papers, homework, short documents | Notes: No \chapter command. Most common for students.
Class: report | Use Case: Longer documents, lab reports | Notes: Adds \chapter. Title page by default.
Class: book | Use Case: Books, theses | Notes: Two-sided. Parts, chapters, front/back matter.
Class: beamer | Use Case: Presentations | Notes: Slide-based. Separate syntax for frames.
Class: letter | Use Case: Formal letters | Notes: Specialized formatting commands.
Class: memoir | Use Case: Flexible long documents | Notes: Superset of book with extensive customization.
Class options go in square brackets:

\documentclass[11pt,a4paper,twoside]{book}
%              ^^^^^  ^^^^^^  ^^^^^^^
%              font   paper   two-sided printing

Common options:

Font size: 10pt (default), 11pt, 12pt
Paper: a4paper, letterpaper, a5paper
Sides: oneside (default for article), twoside (default for book)
Columns: onecolumn (default), twocolumn
Equation numbering: fleqn (left-aligned), leqno (left-numbered)

The Preamble

Everything between \documentclass and \begin{document} is the preamble. This is where you:

Load packages with \usepackage{...}
Define colors, commands, and environments
Set document metadata (title, author, date)
Configure page layout, headers, and fonts

The preamble is configuration. No visible output is produced here.

The Document Body

Everything between \begin{document} and \end{document} is the body. This is your content: text, equations, figures, tables, and references.

Anything after \end{document} is ignored by the compiler.

Packages

Packages extend LaTeX’s capabilities. You load them in the preamble:

\usepackage{amsmath}          % Advanced math
\usepackage[margin=1in]{geometry}  % Page margins
\usepackage{graphicx}         % Images
\usepackage[colorlinks]{hyperref}  % Clickable links

Some packages take options in square brackets. The order of \usepackage declarations usually doesn’t matter, with one major exception: hyperref should be loaded last (or near-last) because it redefines many internal commands.

Important: Essential packages for students:

amsmath — math environments

amssymb — math symbols

graphicx — images

hyperref — links and bookmarks

geometry — page margins

enumitem — list customization

booktabs — professional tables

xcolor — colors

Sectioning

LaTeX provides a hierarchy of sectioning commands:

Command: \part | Level: -1 | Available in: book, report
Command: \chapter | Level: 0 | Available in: book, report
Command: \section | Level: 1 | Available in: all classes
Command: \subsection | Level: 2 | Available in: all classes
Command: \subsubsection | Level: 3 | Available in: all classes
Command: \paragraph | Level: 4 | Available in: all classes
Command: \subparagraph | Level: 5 | Available in: all classes
Numbered vs. Unnumbered Sections

Add an asterisk to suppress numbering:

\section{Introduction}     % Numbered: 1. Introduction
\section*{Acknowledgments} % Unnumbered

This produces:

1 Introduction

Acknowledgments

Starred sections don’t appear in the table of contents by default. To add them manually:

\section*{Acknowledgments}
\addcontentsline{toc}{section}{Acknowledgments}

Labels and Cross-References

One of LaTeX’s most powerful features is automatic cross-referencing. You label things, then refer to them by label. If numbering changes, references update automatically.

\section{Introduction}
\label{sec:intro}

As discussed in Section~\ref{sec:intro} on
page~\pageref{sec:intro}...

\begin{equation}
  E = mc^2
  \label{eq:einstein}
\end{equation}

Equation~\ref{eq:einstein} shows...

Tip: Use a consistent naming convention for labels:

sec: for sections

eq: for equations

fig: for figures

tab: for tables

ch: for chapters

thm: for theorems

This makes labels easy to find and avoids collisions.

Warning: Cross-references require two compilations. After the first pass, references appear as ??. The second pass reads the .aux file and fills them in. If you see ?? in your PDF, compile again.

Front Matter, Main Matter, Back Matter

The book and report classes support three document regions:

\begin{document}

\frontmatter        % Roman numeral pages (i, ii, iii...)
\include{preface}   % No chapter numbers
\tableofcontents

\mainmatter         % Arabic pages (1, 2, 3...)
\include{ch01}      % Chapter 1, etc.
\include{ch02}

\backmatter         % Arabic pages continue
\include{bibliography}  % No chapter numbers
\include{index}

\end{document}

\frontmatter: Roman numeral page numbers, unnumbered chapters (preface, acknowledgments, table of contents).
\mainmatter: Arabic page numbers restart at 1, chapters are numbered.
\backmatter: Arabic numbers continue, chapters are unnumbered (bibliography, index, appendices).

`\input` vs. `\include`

For multi-file projects (like this book), you split content across files:

\input{file}

Simple text insertion
Can be nested
No page break
Works anywhere
Use for: preambles, small sections

\include{file}

Starts a new page
Cannot be nested
Creates separate .aux file
Works with \includeonly
Use for: chapters

The practical difference: use \input for your preamble and small reusable pieces. Use \include for chapters when you want \includeonly support (compile only specific chapters during writing).

\documentclass{book}
\includeonly{ch03,ch04}  % Only compile these chapters
\begin{document}
\include{ch01}  % Skipped, but cross-refs preserved
\include{ch02}  % Skipped
\include{ch03}  % Compiled
\include{ch04}  % Compiled
\end{document}

This book uses \input for everything because we’re using xelatex (which doesn’t benefit from \includeonly‘s auxiliary file caching as much) and we want chapters to flow without forced page breaks between them and the preceding part page.

Comments

Everything after a % on a line is a comment. The compiler ignores it.

This is visible text. % This is a comment.
% This entire line is a comment.

For multi-line comments, use the comment package:

\usepackage{comment}
...
\begin{comment}
Everything in here is ignored.
Even \LaTeX{} commands.
\end{comment}

Whitespace and Line Breaks

LaTeX handles whitespace differently from word processors:

Multiple spaces are treated as a single space.
A single line break is treated as a space.
A blank line (two consecutive line breaks) starts a new paragraph.
\\ forces a line break within a paragraph (use sparingly).
\newpage or \clearpage forces a page break.

These    words    have
multiple   spaces.

This is a new paragraph
because of the blank line.

Manual line break:\\
second line.

This produces:

These words have multiple spaces.

This is a new paragraph because of the blank line.

Manual line break:
second line.

Warning: Don’t use \\ to create space between paragraphs. Use blank lines for paragraph breaks and \vspace{1em} if you need extra vertical space. Abusing \\ creates inconsistent spacing.

Special Characters

Ten characters have special meaning in LaTeX:

Char: # | Meaning: Parameter | Char: % | Meaning: Comment
Char: $ | Meaning: Math mode | Char: ^ | Meaning: Superscript
Char: & | Meaning: Alignment | Char: _ | Meaning: Subscript
Char: { | Meaning: Group open | Char: } | Meaning: Group close
Char: ~ | Meaning: Non-breaking space | Char: \ | Meaning: Command prefix
To typeset these characters literally:

\# \$ \% \& \_ \{ \}
\^{} \~{} \textbackslash

A Complete Starter Template

Here’s a practical template for homework assignments:

\documentclass[11pt]{article}
\usepackage[margin=1in]{geometry}
\usepackage{amsmath,amssymb}
\usepackage{enumitem}
\usepackage{hyperref}

\title{Math 201: Homework 3}
\author{Your Name}
\date{\today}

\begin{document}
\maketitle

\section*{Problem 1}
Find the derivative of $f(x) = x^3 \sin(x)$.

\textbf{Solution.}
Using the product rule:
\begin{align}
  f'(x) &= 3x^2 \sin(x) + x^3 \cos(x) \\
         &= x^2(3\sin(x) + x\cos(x))
\end{align}

\section*{Problem 2}
Evaluate $\displaystyle\int_0^1 e^{-x^2}\,dx$.
% Continue...

\end{document}

Exercises

Create a document using the article class with three sections: Introduction, Methods, and Results. Add a subsection to each. Compile and verify the numbering.

Add labels to each section and create a paragraph that references all three sections by number.

Create a multi-file project: a main file that uses \input to include a separate file for each section. Compile and verify it produces identical output to the single-file version.

Modify the homework template above to include your name, a real problem from one of your classes, and a formatted solution. Compile with xelatex.

The Anatomy of a .tex File