What an index is
An index is an alphabetical list of the names, topics and ideas in a book, each followed by the page numbers where they are discussed. It almost always sits in the back matter, after the main text and any appendices. It is a feature of the book’s structure, much like the ones covered in our overview of the parts of a book.
A simple entry might look like this:
Climate change
economic effects, 122-134
policy responses, 201-218The main entry names the subject, and the indented lines (subentries) point to specific discussions of it.
Why an index matters to readers
An index does several jobs at once.
- It saves time. Instead of flipping through chapters, you look up a name or topic and go straight to the page.
- It helps you judge a book. Browsing the index in a shop or library can show whether a book covers the subject you care about, and how deeply.
- It helps you find things again. After finishing a book, you may remember reading about a person or idea but not where. The index gets you back to it.
- It reveals how ideas connect. Cross-references and subentries show how topics relate, which can make a dense book easier to study.
- It supports the author’s credibility. In scholarly and reference books especially, a careful index signals that the book is meant to be used, not only read once.
Some authors and indexers also note that building an index forces a close final read, which can catch inconsistencies such as a name spelled two ways. That is a benefit to the book itself, even if readers never see the process.
What a good index contains
A strong index reflects concepts, not only words. Professional indexers generally aim for:
- Main entries for key people, places, events and ideas.
- Subentries that break large topics into specific aspects, so readers do not face a long unreadable string of page numbers.
- Cross-references such as “see” and “see also”, which help readers who search under a different term than the author used.
- Consistent wording so the same idea is not listed under several spellings.
One working author describes a rough rule of thumb that an entry should not have more than about six page references before it is split into subentries. Practices differ between publishers, but the underlying idea is the same: an index is only useful if you can tell which page you actually need.
A concordance, which lists every occurrence of a word, is not the same thing. It would be nearly useless for a word like “climate” in a book about climate change.
Index vs table of contents
| Table of contents | Index | |
|---|---|---|
| Position | Front of the book | Back of the book |
| Order | Order of chapters | Alphabetical |
| Detail | Broad chapters and sections | Specific names, terms and ideas |
| Best for | Seeing the book’s structure | Looking up a particular item |
Use the table of contents to understand how the book is organized, and the index to track down a detail.
Index vs glossary
A glossary defines terms. An index only tells you where terms appear. A glossary usually lists unfamiliar or specialized words with short definitions, often in a single place near the back. An index covers a much wider range of material, including names, places and themes, and gives page numbers instead of explanations.
Some books have both, and they work well together: the glossary explains a term, and the index shows every place it is discussed.
Does fiction need an index?
Most novels do not have an index, because readers move through them in order and rarely need to look up a topic. Indexes are far more common in:
- Academic and scholarly books
- History, biography and other narrative nonfiction, including business titles such as those on our best MBA books list
- Textbooks and reference works
- Cookbooks, manuals and how-to guides
Companion volumes for fiction series, such as encyclopedias of a fictional world, often do include one. See our guide to companion books for more on those.
Indexes and e-books
E-readers let you search for any word, which has led some people to ask whether indexes are obsolete. Search and an index solve different problems. Search finds the exact string you type. A good index finds the concept, including places where the author used different words, and shows how the discussion is organized. Not all e-books carry a usable index, and print indexes cannot always be converted well, so the experience varies.
What does 10 9 8 7 6 5 4 3 2 1 mean?
This line of numbers, often found on the copyright page, is called a printer’s key or number line. It indicates which printing of the edition you hold. The lowest number remaining in the line usually shows the printing, so a line that starts at “9” is typically a later printing than one showing “1”. It has nothing to do with the index, but it is another small feature hidden in the front matter that tells you something about the book.
How to use an index well
- Search for the most specific term first, then try broader or related terms.
- Follow “see also” references to find related discussions.
- Look at subentries before opening the page numbers, since they usually tell you which one you need.
- Mark useful entries as you go. If you annotate your own books, our guide to annotation categories for books has ways to do it.
Frequently asked questions
Why is an index helpful?
It lets you locate specific information without reading the whole book, shows what a book covers and helps you return to material later.
What is the definition of an index in a book?
An index is an alphabetical list of topics, names and terms with the page numbers where they occur, usually found at the back of the book.
Is an index the same as a table of contents?
No. The table of contents is at the front and follows the order of the chapters. The index is at the back and is alphabetical and more detailed.
Who writes the index?
Sometimes the author, sometimes a professional indexer hired by the author or publisher. It is generally done after the final page layout is set, so the page numbers are correct.