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An index (plural: usually indexes, see below) is a list of words or phrases ('headings') and associated pointers ('locators') to where useful material relating to that heading can be found in a document. In a traditional back-of-the-book index the headings will include names of people, places and events, and concepts selected by a person as being relevant and of interest to a possible reader of the book. The pointers are typically page numbers, paragraph numbers or section numbers. In a library catalog the words are authors, titles, subject headings, etc., and the pointers are call numbers. Internet search engines, such as Google, and full text searching help provide access to information but are not as selective as an index, as they provide non-relevant links, and may miss relevant information if it is not phrased in exactly the way they expect.
Earliest examples in English
Therefore, even as an index to a book
So to his mind was young Leander's look.
And in such indexes, although small pricks
To their subsequent volumes, there is seen
The baby figure of the giant mass
Of things to come at large.
But according to G. Norman Knight, "at that period, as often as not, by an 'index to a book' was meant what we should now call a table of contents."
Among the first indexes – in the modern sense – to a book in the English language was one in Plutarch's Parallel Lives, in Sir Thomas North's 1595 translation. A section entitled "An Alphabetical Table of the most material contents of the whole book" may be found in Henry Scobell's Acts and Ordinances of Parliament of 1658. This section comes after "An index of the general titles comprised in the ensuing Table". Both of these indexes predate the index to Alexander Cruden's Concordance (1737), which is erroneously held to be the earliest index found in an English book.
Terminology: indexes v indices
"But the real importance of this passage is that it establishes for all time the correct literary plural; we can leave the Latin form "indices" to the mathematicians (and similarly "appendices" to the anatomists)."
The term "indexes" is used throughout the publishing industry, including in the International Standard ISO 999 - Information and documentation - Guidelines for the content, organization and presentation of indexes.
The indexer reads through the text, identifying indexable concepts (those for which the text provides useful information and which will be of relevance for the text's readership). The indexer creates index headings to represent those concepts, which are phrased such that they can be found when in alphabetical order (so, for example, one would write 'indexing process' rather than 'how to create an index'). These headings and their associated locators (indicators to position in the text) are entered into specialist indexing software which handles the formatting of the index and facilitates the editing phase. The index is then edited to impose consistency throughout the index.
Indexers must analyze the text to enable presentation of concepts and ideas in the index that may not be named within the text. The index is intended to help the reader, researcher, or information professional, rather than the author, find information, so the professional indexer must act as a liaison between the text and its ultimate user.
There are several dedicated, indexing software programs available to assist with the special sorting and copying needs involved in index preparation. The most widely known include Cindex, Macrex and SkyIndex. TExtract is a hybrid semi-automatic program combining conventional manual indexing with automated indexing features and text linking.
Embedded indexing involves including the index headings in the midst of the text itself, but surrounded by codes so that they are not normally displayed. A usable index is then generated automatically from the embedded text using the position of the embedded headings to determine the locators. Thus, when the pagination is changed the index can be regenerated with the new locators.
LaTeX documents support embedded indexes primarily through the MakeIndex package. Several widely used XML DTDs, including DocBook and TEI, have elements that allow index creation directly in the XML files. Most word processing software, such as StarWriter/OpenOffice.org Writer, Microsoft Word, and WordPerfect, as well as some desktop publishing software (for example, FrameMaker and InDesign), as well as other tools (MadCap Software's Flare)) have some facility for embedded indexing as well. TExtract supports embedded indexing of Microsoft Word documents.
An embedded index requires more time to create than a conventional static index; however, an embedded index can save time in the long run when the material is updated or repaginated. This is because, with a static index, if even a few pages change, the entire index must be recreated, while with an embedded index, only the pages that changed need updating or indexing.
Indexes are designed to help the reader find information quickly and easily. A complete and truly useful index is not simply a list of the words and phrases used in a publication (which is properly called a concordance), but an organized map of its contents, including cross-references, grouping of like concepts, and other useful intellectual analysis.
Sample back-of-the-book index excerpt:
- sage, 41-42. See also Herbs ← directing the reader to related terms
- Scarlet Sages. See Salvia coccinea ← redirecting the reader to term used in the text
- shade plants ← grouping term (may not appear in the text; may be generated by indexer)
- hosta, 93 ← subentries
- myrtle, 46
- Solomon's seal, 14
- sunflower, 47 ← regular entry
In books, indexes are usually placed near the end (this is commonly known as "BoB" or back-of-book indexing). They complement the table of contents by enabling access to information by specific subject, whereas contents listings enable access through broad divisions of the text arranged in the order they occur. It has been remarked that, while "[a]t first glance the driest part of the book, on closer inspection the index may provide both interest and amusement from time to time." 
Some principles of good indexing include:
- Ensuring each topic/section includes a variety of relevant index entries; use two or three entries per topic
- Understanding the audience and understand what kind of index entries they're likely to look for
- Use the same form throughout (singular vs. plural, capitalisation, etc.), using standard indexing conventions
- Significant topics with no index entries at all
- Indexing 'mere mentions' --- "But John Major was no Winston Churchill..." indexed under 'Churchill, Winston'
- Circular cross-references: 'Felidae. See Cats' --- 'Cats. See Felidae'
- References to discussions of a single topic scattered among several main headings: 'Cats, 50-62' --- 'Felidae, 175-183'
- Inconsistently indexing similar topics
- Confusing similar names: Henry V of England, Henri V of France
- Incorrect alphabetization: 'α-Linolenic acid' under 'A' instead of 'L'
- Inappropriate inversions: 'processors, word' for 'word processors'
- Inappropriate subheadings: 'processors: food, 213-6; word, 33-7'
- Computer indexing from section headings: e.g. 'Getting to know your printer' under 'G'
Some indexers specialize in specific formats, such as scholarly books, microforms, web indexing (the application of a back-of-book-style index to a website or intranet), search engine indexing, database indexing (the application of a pre-defined controlled vocabulary such as MeSH to articles for inclusion in a database), and periodical indexing (indexing of newspapers, journals, magazines).
Some indexers specialize in particular subject areas, such as anthropology, business, computers, economics, education, government documents, history, law, mathematics, medicine, psychology, and technology. An indexer can be found for any subject.
References in popular culture
Kurt Vonnegut's novel Cat's Cradle includes a character who is a professional indexer and believes that "indexing [is] a thing that only the most amateurish author [undertakes] to do for his own book." She claims to be able to read an author's character through the index he created for his own history text, and warns the narrator, an author, "Never index your own book."
- ISO 999:1996 Guidelines for the Content, Organization, and Presentation of Indexes (this is also the national standard in the UK, Australia, and New Zealand)
- Indexing Society of Canada 
- American Society for Indexing 
- Australian and New Zealand Society of Indexers 
- China Society of Indexers 
- Society of Indexers 
- Nederlands Indexers Netwerk (NIN)
- British Record Society
- Back-of-the-book index
- Concordance (publishing)
- Indexing and abstracting service
- Subject indexing
- Web indexing
- Human or computer produced indexes?
- Knight, G. Norman (1979) Indexing, the Art of: A Guide to the Indexing of Books and Periodicals (HarperCollins), pp. 17–18
- Robert L. Collison, Book Collecting, London, 1957, p. 121.
- http://www.adobe.com/devnet/robohelp/articles/online_help_pt2_06.html, Creating Online Help (Part 2): Strategies and Implementation
- Weaver, Carolyn. "The Gist of Journal Indexing", Key Words 10.1 (Jan./Feb. 2002), 16–22.
- Booth, Pat (2001) Indexing: The Manual of Good Practice (K. G. Saur), ISBN 3-598-11536-9
- Borko, Harold & Bernier, Charles L. (1978) Indexing Concepts and Methods, ISBN 0-12-118660-1
- Browne, Glenda and Jermey, Jon (2007), The Indexing Companion (Cambridge University Press), ISBN 978-0-521-68988-5
- Mulvany, Nancy (2005) Indexing Books, 2nd ed. (University of Chicago Press) ISBN 0-226-55276-4
- Smith, Sherry & Kells, Kari (2005) Inside Indexing: the Decision-Making Process (Northwest Indexing Press), ISBN 0-9771035-0-1
- Stauber, Do Mi (2004) Facing the Text: Content and Structure in Book Indexing (Cedar Row Press) ISBN 0-9748345-0-5
- Wellisch, Hans (1995) Indexing from A to Z, 2nd ed. (H. W. Wilson) ISBN 0-8242-0807-2
- The Indexer (the international journal)
- Consideration in Indexing Online Documents
- Usability studies for indexes
- Reflections on Authorship and Indexing
- "The Definite Article: Acknowledging 'The' in Index Entries", Glenda Browne, The Indexer, vol. 22, no. 3 April 2001, pp. 119-22.
- The role of indexing in technical communication
- Indexing FAQ/Intro
- Author-created indexes