Headings that number themselves

In a word processor, you number headings by hand and renumber them by hand every time you move one. LaTeX refuses to let you do that. You declare what each heading is, a section or a subsection or a chapter, and the numbering takes care of itself. Insert a new section in the middle of your document and everything after it renumbers on the next compile, along with every cross-reference and every table of contents entry that points at it.

The hierarchy

\part{...} % book
\chapter{...} % report/book
\section{...}
\subsection{...}
\subsubsection{...}
\paragraph{...}

These commands form a fixed ladder. \section is the everyday workhorse in the article class, with \subsection and \subsubsection nesting beneath it. \chapter only exists in the report and book classes, which is why pasting thesis chapters into an article template fails with an undefined control sequence. \part sits above chapters and is rare outside long books. \paragraph is the odd one out: it produces a bold run-in heading on the same line as the following text rather than a heading on its own line, which surprises almost everyone the first time.

Each command takes the heading text in braces, prints it in the class’s chosen font and size, numbers it, and records it so \tableofcontents can find it later. You never style headings inline; if you want different heading fonts, that is a document-wide decision made in the preamble.

Suppressing numbers

Starred forms suppress numbers: \section*{Acknowledgments}. The star also keeps the heading out of the table of contents and out of the numbering sequence entirely, so the section after it continues as if the starred one were not there. This is exactly what you want for acknowledgments, a preface, or a standalone conclusion in a short paper. If a starred section should still appear in the TOC, there is a one-line fix covered in the TOC lesson.

Controlling numbering depth

\setcounter{secnumdepth}{2}
\setcounter{tocdepth}{2}

secnumdepth decides how deep numbering goes. In article, sections are level 1, subsections level 2, and subsubsections level 3, so setting the counter to 2 numbers sections and subsections but leaves subsubsection headings unnumbered while keeping their formatting. tocdepth is the same idea for the table of contents: it controls how deep the TOC listing goes, independently of what gets numbered on the page. Both lines belong in the preamble.

A common mistake is skipping levels, for example jumping from \section straight to \subsubsection because you like the smaller heading. The numbering still works but produces odd results like “1.0.1”, and readers lose track of the structure. Pick heading levels for meaning and let the class handle the appearance. If you are unsure how your document’s skeleton fits together, the document skeleton lesson walks through it.