# How headers and sections map onto to book structure#

Jupyter Book uses the Sphinx documentation engine under the hood, which represents the structure of your book in a particular way. Different choices for the structure of _toc.yml and the header structures within your pages will result in different outcomes for your overall book structure. Here are some general tips and best-practices.

Note

This is particularly important when you number your book’s sections or when you build a PDF of your book through Latex.

Chapters are at the top of your book hierarchy. The top level of your _toc.yml contains a list of chapters. The title of each file will be the chapter’s title.

Headers map onto sections. Jupyter Book interprets your book as a collection of sections, and decides how those sections should be nested according to the hierarchy of _toc.yml and the hierarchy of headers in a page. Within a file, the first ##  header it discovers will define the top-most section in the file, and any subsequent ###  headers underneath will become sub-sections (until another ##  section is encountered). This behavior is a bit different if the page is nested under another (see below).

Nested files define sections underneath the last section of their parent. If you specify sections that are nested under a file (with the sections: key) then those sections will begin underneath the last headers of the parent page.

For example, if your _toc.yml file looks like this:

format: jb-book
root: myintro
chapters:
- file: chapter1
sections:
- file: chapter1section


Then the sections of chapter1section will begin under the sections of chapter1. Any headers in chapter1section will be treated as a “next-header-deeper” section in chapter1.

In other words, if chapter1 and chapter1section look like this:

chapter1.md

# Chapter 1 title



chapter1section.md

# Chapter 1 section title

## Chapter 1 section second header


Then your book will treat them like so:

# Chapter 1 title

### Chapter 1 section title

#### Chapter 1 section second header


However, if chapter1.md had an extra third-level header, like so:

chapter1.md

# Chapter 1 title



chapter1section.md

# Chapter 1 section title

## Chapter 1 section second header


Then your book would treat them like so:

# Chapter 1 title

#### Chapter 1 section title

##### Chapter 1 section second header


Keep this in mind when you design the structure of your files.

Tip

A good rule of thumb is to take one of these two approaches:

1. don’t put headers in your introduction pages. This is true for both the book’s introduction, as well as for any chapter introductions. Instead, leave the headers to pages that have more content in them, and use bolded text where you would otherwise use headers.

2. Use a flat list of files instead of nested files. This way the section hierarchy is defined only in a single file within each section. However, this means you will have longer files in general.