Indexes or Indices | Meaning, Usage and Grammar Explained In 2026

Quick Answer

Indexes and indices are both correct plurals of “index.” Use indexes for general English, especially book or database lists, and indices more often in mathematical, scientific, and technical contexts.

If you have ever wondered whether the plural of index is indexes or indices, you are not alone. Both forms are correct, but they are often used in different contexts. Indexes is common in everyday English and can refer to lists, databases, books, or other collections of information. Indices, on the other hand, is especially common in technical, mathematical, scientific, and financial writing. Understanding this distinction can help you choose the form that sounds natural and fits your subject.

The choice between indexes and indices is not simply a matter of one word being right and the other being wrong. English has adopted both forms over time, and their usage depends largely on context, audience, and field. In this guide, we’ll explain what each plural means, show clear examples, and help you understand when to use indexes, indices, or either form interchangeably.

Which One Is Correct?

Both “Indexes” and “Indices” Are Correct

There is no simple right-versus-wrong answer here. Indexes and indices are both established plural forms of the singular noun index.

The difference is mostly one of usage, tradition, and context, rather than grammar.

For example:

• “The book contains several indexes.”
• “The mathematical formula uses several indices.”
• “The database stores multiple indexes.”
• “Researchers compared several economic indices.”

All four sentences can be grammatically correct.

The important question is not simply, “Which spelling is correct?” Instead, ask:

What kind of index am I talking about?

If you mean lists at the back of books, database structures, or general categories, indexes is often the natural choice.

If you are discussing mathematical variables, scientific measurements, statistical indicators, or technical notation, indices is often preferred.

Why Are There Two Plurals?

English has borrowed many words from Latin and other languages. Some borrowed nouns have kept an older plural form, while English has also created a regular plural ending in -s or -es.

That is what happened with index.

The traditional Latin plural is indices, while English developed the regular plural indexes.

Over time, both became established in English.

This is not unusual. English contains many nouns with more than one accepted plural form, particularly words that entered the language from Latin or Greek.

Meaning of “Indexes”

What Does “Indexes” Mean?

Indexes is the plural of index formed according to ordinary English spelling patterns.

An index can mean a list, a reference system, an indicator, or a numerical measure. Therefore, indexes can refer to several kinds of collections or systems.

For example:

• “The library uses alphabetical indexes.”
• “The database contains several indexes.”
• “The publisher created separate subject indexes.”
• “Search engines rely on enormous data indexes.”

The word is particularly natural when index means a list that helps people locate information.

Indexes in Books

One of the most familiar meanings of index is the alphabetical list found near the end of a book.

A book may have separate indexes for:

• Subjects
• Names
• Places
• Terms
• Authors
• Topics

For example:

“The textbook includes detailed indexes of names and subjects.”

Here, indexes is a very natural choice because we are talking about reference lists.

Indexes in Databases

The word also appears in computing and database management.

A database index is a structure that helps a system find information more efficiently.

When there is more than one, indexes is common:

“The database uses several indexes to speed up searches.”

In technical writing, you may also encounter indices, but indexes is widely used in computing.

Indexes as Indicators

An index can also be a numerical indicator of a condition or trend.

For example:

“Several price indexes were published during the year.”

Here, indexes is perfectly acceptable, although indices is also common in economic and statistical writing.

This is one area where the two forms can overlap.

Meaning of “Indices”

What Does “Indices” Mean?

Indices is traditionally derived from the Latin plural of index. In modern English, it is especially common in specialized fields.

You are likely to encounter indices in:

• Mathematics
• Physics
• Chemistry
• Engineering
• Statistics
• Economics
• Finance
• Scientific research

For example:

“The equation contains three indices.”

Or:

“Several economic indices were compared.”

The form sounds more formal or technical in many contexts.

Indices in Mathematics

Mathematics is one of the clearest areas where indices is common.

An index can identify a member of a sequence, array, set, or mathematical object.

For example:

“The values are labeled using different indices.”

In some mathematical contexts, an index is a number or symbol used to distinguish one item from another.

Consider a sequence:

a₁, a₂, a₃, a₄

The numbers 1, 2, 3, and 4 can be described as indices.

A mathematical text might say:

“Let i and j be two indices.”

Here, indices sounds natural and is widely recognized.

Indices in Science

Scientific writing also frequently uses indices.

A scientific index can be a numerical measure used to represent a property, condition, or relationship.

For example:

“The researchers compared several environmental indices.”

Or:

“The study examined indices related to plant growth.”

In scientific contexts, the choice of indices often reflects the technical tradition of the field.

Indices in Economics and Finance

Economics frequently uses the word index for measurements such as price levels, market performance, or economic conditions.

As a result, both plural forms can appear.

Examples include:

• Consumer price indices
• Stock market indices
• Economic indices
• Production indices

For example:

“The latest economic indices showed a change in consumer activity.”

However, indexes can also be correct in economic writing. The choice may depend on the publication, institution, or regional convention.

Key Differences Between “Indexes” and “Indices”

Spelling

The most obvious difference is the ending.

Index → indexes

Index → indices

The first uses the regular English plural ending -es.

The second preserves the traditional Latin-derived plural form.

Meaning

In most cases, there is no major difference in basic meaning.

Both words can refer to more than one index.

The distinction usually comes from context rather than from a completely different definition.

For instance:

• “book indexes”
• “mathematical indices”

Both describe multiple indexes, but the surrounding subject influences which plural sounds more natural.

Pronunciation

The two forms are pronounced differently.

Indexes is commonly pronounced approximately:

IN-dek-siz

The ending sounds like -siz.

Indices is commonly pronounced approximately:

IN-di-seez

The final part sounds like -seez.

So the difference is easy to hear when the words are spoken aloud.

Grammar

Both are plural nouns.

The singular form is:

index

The plural forms are:

indexes / indices

Examples:

• “This is an index.”
• “These are indexes.”
• “These are indices.”

Neither plural requires a different singular form.

Usage

The biggest difference is usage.

This table gives a useful general pattern, but it is not an absolute rule.

Recognition

Indexes is often easier for general readers because it follows the familiar English plural pattern.

Indices may sound more specialized because it retains a traditional plural form.

For that reason, a general-audience writer may naturally prefer indexes, while a mathematician or scientist may choose indices.

When Should You Use “Indexes”?

Use “Indexes” for Book Lists

When discussing reference lists in books, indexes is usually the safest choice.

Examples:

“The encyclopedia has separate indexes for people and places.”

“The publisher added detailed indexes to the revised edition.”

Use “Indexes” in General Writing

If you are not writing in a specialized field, indexes is often the straightforward option.

Examples:

“The website has indexes of important terms.”

“Several indexes were created to organize the archive.”

Use “Indexes” in Computing

In database and programming contexts, indexes is very common.

Example:

“The administrator created indexes for the most frequently searched fields.”

This form is widely understood by technical readers.

When Should You Use “Indices”?

Use “Indices” in Mathematics

When index refers to mathematical notation or a position in a mathematical structure, indices is often the preferred form.

Example:

“The equation contains two indices.”

Use “Indices” in Scientific Writing

In many scientific fields, indices sounds natural and conventional.

Example:

“The researchers calculated several ecological indices.”

Use “Indices” for Technical Measurements

When several numerical indicators are being discussed, indices is often appropriate.

Examples:

• “health indices”
• “economic indices”
• “performance indices”
• “environmental indices”

However, the exact convention can vary between fields.

Can “Indexes” and “Indices” Be Interchanged?

Sometimes, But Not Always Naturally

In many sentences, you can technically replace one with the other without creating a grammatical error.

For example:

“The report includes several economic indexes.”

and

“The report includes several economic indices.”

Both can be found in reputable English usage.

However, one may sound more natural to readers familiar with a particular field.

A mathematician may expect indices.

A database developer may expect indexes.

An editor preparing a book may naturally write indexes.

Therefore, the choice is often about convention and audience, not correctness.

Follow the Convention of Your Field

If you are writing for a specialized audience, consistency matters.

For example, if a mathematical textbook consistently uses indices, switching randomly between indexes and indices can make the writing look careless.

Likewise, if a database manual consistently uses indexes, there is little reason to change every occurrence to indices.

Once you choose the form that fits your context, use it consistently.

Common Mistakes People Make

Mistake 1: Thinking “Indices” Is Always More Correct

Some writers assume that because indices comes from Latin, it must be the “proper” plural.

That is not true.

Indexes is a fully accepted English plural.

There is no need to replace indexes with indices simply to sound more formal.

Mistake 2: Thinking “Indexes” Is Incorrect

The opposite mistake is also common.

Some people learn that index has the plural indices and conclude that indexes is wrong.

It is not.

Modern English dictionaries and usage recognize indexes as a valid plural.

Mistake 3: Using “Indices” Everywhere

Using indices in every situation can make ordinary writing sound unnecessarily technical.

For example:

“The book has three indices.”

This is understandable, but if you mean three alphabetical reference lists, indexes may sound more natural.

Mistake 4: Using “Indexes” in Highly Technical Writing Without Checking Convention

Although indexes is correct, a specialized field may traditionally favor indices.

For example:

“The variables are represented by different indices.”

In mathematical writing, indices is generally the more familiar choice.

Mistake 5: Confusing “Indices” With “Index’s”

These are completely different forms.

Indices = plural

Index’s = possessive singular

For example:

“The index’s purpose is to organize the information.”

But:

“The indices contain different categories.”

Mistake 6: Confusing “Indices” With “Indicies”

Indicies is a common spelling mistake.

The correct plural is:

indices

Not:

indicies

The spelling follows the traditional form index → indices.

Correct Usage Examples

Casual Examples

• “The book has several indexes.”
• “I checked the indexes at the back of the textbook.”
• “The website provides indexes for different topics.”
• “Which indexes should I use to find the information?”

Professional Examples

• “The report contains separate indexes for each category.”
• “The database uses multiple indexes to improve search performance.”
• “The researchers compared several economic indices.”
• “The analysts reviewed the latest market indices.”

Educational Examples

• “The textbook contains useful indexes for students.”
• “Mathematical indices can identify different elements in a sequence.”
• “The equation uses two indices to represent the variables.”
• “Students should check the indexes when looking for a specific topic.”

Scientific Examples

• “The study evaluated several environmental indices.”
• “The researchers calculated health-related indices.”
• “Different indices were used to measure changes over time.”
• “The values were compared across several scientific indices.”

Literary and Metaphorical Examples

Although index is often technical, it can also be used more broadly.

Examples:

“The scars became indexes of the difficult years he had survived.”

“The old photographs served as indexes of a changing town.”

In these creative contexts, indexes is usually easier and more natural for general readers.

Indexes or Indices in Different Fields

Books and Publishing

Indexes is generally the natural choice.

Example:

“The reference book contains subject and author indexes.”

The word refers to organized lists that help readers locate information.

Mathematics

Indices is generally preferred.

Example:

“The formula uses different indices to distinguish the variables.”

This is a technical use of the word.

Computer Science

Indexes is very common, particularly when discussing databases.

Example:

“The database uses indexes to retrieve records efficiently.”

Economics

Both forms occur.

Examples:

“Economic indexes are used to track changes.”

“Economic indices are used to compare performance.”

The preferred form can depend on the writer, institution, or regional style.

Finance

Both indexes and indices are used for financial indicators and market measures.

For example, you may encounter:

• Stock market indexes
• Stock market indices
• Price indexes
• Price indices

Neither form should automatically be treated as an error.

Word Origin and Etymology

Where Does “Index” Come From?

The English word index comes from Latin index, meaning something like an informer, pointer, indicator, or one who points out.

The Latin noun is related to the verb indicāre, meaning “to point out” or “to indicate.”

This connection makes sense when you consider the modern meaning of index. An index helps point you toward information.

A book index points readers toward specific pages.

A numerical index indicates a particular measurement or trend.

A mathematical index identifies a particular element or position.

Why Does “Indices” Exist?

Indices is based on the traditional Latin plural form.

English borrowed many Latin words while also adapting them to normal English grammar. As a result, some words developed both a traditional plural and an English plural.

Index is one of them.

The Latin-derived plural indices survived in English, especially in technical and academic language.

At the same time, English speakers naturally formed indexes using the familiar plural ending.

Both forms eventually became established.

Why Did “Indexes” Become So Common?

English Prefers Simple Plural Patterns

English commonly creates plurals by adding -s or -es.

Examples include:

• box → boxes
• class → classes
• tax → taxes
• index → indexes

Because indexes follows this familiar pattern, it is easy for English speakers to form and understand.

Different Fields Developed Different Habits

Specialized communities often preserve older terminology.

Mathematicians, scientists, and academics have traditionally used many Latin-derived terms. This helped indices remain common in technical writing.

Meanwhile, general English, publishing, and computing often favor indexes.

This is why both forms remain in use today.

Easy Memory Tricks

Think “Books = Indexes”

If you are talking about lists at the back of books, remember:

Books have indexes.

For example:

“Check the book indexes for the author’s name.”

Think “Math = Indices”

For mathematical notation, remember:

Math uses indices.

For example:

“The variables are identified by indices.”

This is not an unbreakable rule, but it is an easy guide.

Remember the Spelling

If you want the traditional plural, remember:

index → indices

Do not write indicies.

If you want the regular English plural:

index → indexes

Remember That Both Are Correct

The easiest memory trick of all is:

Indexes is not wrong, and indices is not automatically better.

Your context decides which form is most appropriate.

A Simple Decision Guide

When you are unsure which plural to use, ask yourself three questions.

Question 1: Am I Talking About a Book or General List?

If yes, indexes is usually a good choice.

“The book contains several indexes.”

Question 2: Am I Writing About Mathematics or Science?

If yes, indices will often be the more natural choice.

“The equation contains several indices.”

Question 3: Am I Writing About Economics or Finance?

If yes, check the convention used by your field or publication.

Both indexes and indices are used in these areas.

Indexes vs. Indices at a Glance

FAQs

Is “indexes” a correct plural of “index”?

Yes. Indexes is a standard and correct English plural of index. It is especially common in general writing, publishing, and computing.

Is “indices” a correct plural of “index”?

Yes. Indices is also correct. It is especially common in mathematics, science, statistics, economics, and other technical contexts.

Which is better, indexes or indices?

Neither is universally better. Indexes is usually more natural in general English, while indices is often preferred in mathematical and scientific writing.

Is “indicies” the correct spelling?

No. Indicies is a misspelling. The correct traditional plural is indices, while the regular English plural is indexes.

What is the plural of index in mathematics?

Indices is the traditional and widely preferred plural in mathematics. For example, “The equation contains several indices.”

What is the plural of index in a book?

Indexes is generally the most natural choice when referring to lists at the back of books. For example, “The book contains separate indexes of names and subjects.”

Are indexes and indices pronounced differently?

Yes. Indexes is commonly pronounced approximately “IN-dek-siz,” while indices is commonly pronounced approximately “IN-di-seez.”

Can I use indexes and indices interchangeably?

Sometimes, especially when referring to numerical indicators, but the preferred form can depend on the field. In general writing, indexes is often the simpler choice; in mathematics and technical subjects, indices is frequently preferred.

Conclusion

Indexes and indices are both correct plural forms of “index,” but they are used in different contexts.
Indexes is common in general English, books, publishing, and computing, while indices is especially familiar in mathematics and science.When writing about economics or finance, either form may be appropriate depending on the convention you follow.The best choice is the one that matches your subject, audience, and established style.

Jessica Morgan

Jessica Morgan is a blogger and writer at GramScript.com, where she turns everyday ideas into engaging, relatable stories. Her writing blends a natural voice with fresh perspectives that keep readers coming back.