What’s the Plural of Index: Understanding Singular and Plural Nouns

  • Index” can be pluralized as “indexes” or “indices” depending on context.
  • Indexes” is commonly used for lists in books, while “indices” in mathematical contexts.
  • Both plural forms are valid and reflect the diversity of English noun plurals.

The word “index” has more than one acceptable plural form, yielding to two main variants: “indexes” and “indices.” The selection between these forms often hinges on context and tradition. “Indexes” is commonly used in reference to alphabetical lists in books and databases, while “indices” is preferred in mathematical and technical contexts to describe multiple sets of numerical figures or indications.

What’s the Plural of “Index”?

When discussing the term “index”, one may encounter two commonly accepted plural forms: indexes and indices. Both plurals are correct, but their usage often depends on context.

Regular Plural Form: Indexes

Indexes is the more commonly used plural form in everyday English, particularly in American English. It aligns with the standard rule of adding “-es” to nouns ending in -x.

  • Example in a sentence:
    • Most libraries have catalogs with multiple indexes to help find books.

Latin Plural Form: Indices

Indices adheres to the original Latin pluralization of the word. This form is predominantly used in mathematical and technical contexts.

  • Example in a sentence:
    • In calculus, one often deals with multiple indices when working with tensors.

Contextual Usage:

  • Mathematics and Technical Fields: Indices
    • The plural of “index” in mathematical and technical language is often “indices”, reflecting its Latin roots.
  • General Use: Indexes
    • In general prose and conversation, “indexes” is the preferred plural form.

“Indices” or “Indexes”: Which is Correct?

In formal and technical contexts, particularly in fields such as mathematics or sciences, “indices” is generally preferred. This plural form adheres to the original Latin. For example, mathematical texts typically reference multiple indexes of equations as “indices.”

In more general and non-technical contexts, “indexes” is widely accepted and often used. Publications and literature unrelated to scientific or technical content typically employ this form.

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ContextPreferred Plural
Formal or TechnicalIndices
General or Non-TechnicalIndexes

Language experts mention that “indices” can be used when referring to databases or referencing systems, while “indexes” suits applications such as library catalogs or book contents.

  • “Indices” commonly references:

    • Mathematical or scientific measurements
    • Database lookup arrays
    • Statistical series within financial markets
  • “Indexes” is often used for:

    • Tables of contents in books
    • Search engine listings
    • Periodical directories

Defining “Index” – What Does it Mean?

An index is a systematic arrangement of materials, often ordered alphabetically, serving as a guide or point of reference. It’s a tool prevalent in books, databases, and search engines to facilitate the retrieval of information. The term originates from Latin, literally meaning “to point out” or “to indicate.”

Forms of Index:

  • Book Index: A list of words or phrases found at the end of books, helping readers locate information.
  • Database Index: Structures that improve database query speed by minimizing the number of disk accesses.
  • Search Engine Index: A collection of information about websites, enabling search engines to return relevant results quickly.

Indexes are essential in an array of fields, including academics, technology, and finance. They allow users to quickly access desired information without having to peruse extensive amounts of material.

Characteristics of an Index:

FunctionAids in finding information
ArrangementTypically alphabetical
AccessibilityUser-friendly and efficient retrieval

Indexes are not to be confused with indices, which can also refer to mathematical or economic contexts. In mathematical terms, indices are exponents, while in financial markets, they represent the performance measurement of a section of shares or stocks.

Index in Technology:

  • Web Index: Used by search engines like Google to rank and locate web pages.
  • Database Index: Often invisible to the end-user, but critical for performance.

Irregular Nouns: -ex/-ix and -ces/-xes Endings

Plurals for -ex/-ix Endings

Nouns ending in -ex or -ix typically replace these endings with -ices or -xes when forming the plural. The following table illustrates some common examples:


However, it’s important to note that some words can also take on a more regular plural form by merely adding -es or changing to -x. This alternative often coexists with the more irregular form:

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SingularPlural (Alternative)

Plurals for -ce/-xe Endings

Now, let’s consider words ending in -ce and -xe. Many of these nouns form their plural by simply adding an -s. Here are a few examples, shown with both their singular and plural forms:

  • Piece becomes Pieces.
  • Price changes to Prices.

In contrast, some nouns require a different approach, adding -es for plurals:

  • Box transforms into Boxes.
  • Fox turns into Foxes.

In most instances, the addition of -s or -es will suffice, but vigilance is key as exceptions do exist.

Examples of “Index” Used in Context

In finance, analysts might say:

  • The Dow Jones Industrial Average is one of the most followed equity indices.
  • Investors keep an eye on stock market indexes to gauge market movements.

In academia, particularly in the sciences, researchers may use:

  • This study compares several different methods for calculating refractive indices.
  • The report lists the chemical properties of materials in its indices for easy reference.

Here, two tables are presented to provide clarity on the use of “index” in different contexts:

SingularPlural (Technical)Plural (General)Example Sentence
indexindicesindexesThe journal has a comprehensive set of indices.
   You can locate the chapter names in the book’s indexes.
ContextWhen to Use “Indices”When to Use “Indexes”
Academic PapersCommonLess Common
Stock MarketsLess CommonCommon
Literary WorksCommon
  • Librarians may discuss how:

    • Indices in old manuscripts require meticulous curation.
    • Esteemed libraries host catalogs with numerous author indexes.
  • In technology and database management, IT professionals often encounter:

    • Database systems have complex indices designed to speed up query times.
    • Software documentation includes indexes for functions and classes.

Examples of “Indices/Indexes” Used in Context

Below are examples displaying how each term can be employed in different scenarios.

Academic and Technical Fields:

  • In mathematics, the plural form “indices” is commonly used to refer to multiple exponents or locators in a mathematical set or sequence.
    Singular Plural
    The index of a series The indices of a series
    An equation’s index Equations with multiple indices
  • In finance, “indices” is often used to talk about multiple stock market indicators.
    Singular Plural
    The stock index Stock market indices
    The consumer price index Consumer price indices
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Publishing and General Use:

  • In publishing or when referring to book indexes or databases, both “indexes” and “indices” are correct and interchangeable.
    • A database might update its indexes weekly.
    • Many reference books include detailed indexes at the end.
    Books with an index Books with multiple indexes
    A dictionary’s list of words Dictionaries with thorough indexes

Computer Science:

  • In the field of computer science, specifically when discussing arrays or lists in programming, “indexes” is preferred in casual contexts, but “indices” can also appear.
    Singular Plural
    The array index Array indexes
    The list index Lists with multiple indexes

Synonyms for “Index”

Here are some common synonyms for “index”:

  • Catalog: Often used in the context of libraries or collections, implying a systematic list of items.
  • List: A general term for any sequence of items, often used interchangeably with “index.”
  • Register: Implies a record or a list, usually official or formal in nature.
  • Directory: A listing, typically of names and addresses, such as a telephone directory.
  • Indicator: Suggests a measure or a sign that indicates the state or level of something.

Usage Contexts

Different synonyms of “index” are preferable depending on the specific use case or field.

ContextPreferred Synonym
Technical DataList
Contact DetailsDirectory

Associations and Connotations

The selection of a synonym for “index” can subtly alter the connotation of the message. For instance, “catalog” might imply extensive organization, suited for large collections or databases.


Origin of the Word “Index”

Key Developments of the Term “Index”:

  • Medieval Era: Initial scholarly usage to refer to alphabetized lists of subjects.
  • Renaissance: Expanded use to reference tables of contents and pointers to knowledge in books.
EraUse of “Index”
MedievalAlphabetized lists to organize information
RenaissanceTables of contents in books; navigating scholarly tomes

During the Renaissance, the printing press popularized books, and the index became an invaluable feature for navigation within texts. It was a period marked by a surge in documentation, amplifying the need for organized referencing systems.

Later Developments:

  • 17th and 18th Centuries: Became more systematic, reflecting the Enlightenment’s emphasis on order.

Modern Usage:
“Index” is now ubiquitously used in a variety of contexts where classification and referencing are crucial. For example, indexes in mutual funds and stock markets are invaluable for investors to track market movements and trends.

ContextUse of “Index”
Libraries and AcademiaOrganizing and finding information within texts
Financial MarketsReflecting the performance of specific stock market segments

Reflecting its rootedness in traditions of organization and clarity, “index” has evolved in meaning but retains its core function of pointing out, indicating, and referencing.


  1. Definition of index finger.
  2. Definition of index.
  3. Synonyms for index.
  4. Index sentence examples.
  5. Origin on index.

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