Indexes vs Indices is a common grammar question because both words are correct plural forms of index, but writers use them in different contexts. In general English, publishing, and SQL databases, indexes is a common choice. In finance, mathematics, and scientific writing, indices often appears as the preferred form. The distinction also matters in Python programming, legal documents, and British and American English.
Understanding the difference helps you choose the right word for your audience without treating a professional convention as a strict grammar rule. Whether you are writing about stock market indices, creating database indexes, explaining Python list positions, or preparing a legal document index, the correct choice depends on context. This guide explains the plural of index, the meaning and pronunciation of indices, and practical examples that show when to use each form.
Indexes vs Indices: The Quick Answer
Both indexes and indices are grammatically correct plurals of index. Use indexes for general English, book references, and database structures when that terminology fits your context. Use indices when discussing stock markets, mathematics, or scientific measurements where the traditional form is standard. Neither plural is universally more correct than the other.
Here are two examples:
- Indexes: The textbook contains detailed indexes of names, places, and technical terms.
- Indices: Several major stock market indices declined after investors reacted to the economic news.
Both sentences use the plural correctly. The difference lies in convention and meaning within a particular field.
| Question | Short answer |
| Is indexes correct? | Yes. It is an established English plural of index. |
| Is indices correct? | Yes. It is another accepted plural of index. |
| What is the plural of index in SQL? | Indexes is the conventional term in SQL documentation. |
| What is the plural of index in finance? | Indices is widely used in financial writing. |
| What is the singular of indices? | Index. |
| Which form should I use in general writing? | Indexes is a straightforward default unless context or a style guide favors indices. |
The easiest rule to remember is this: books and databases commonly use indexes, while financial markets and mathematics commonly use indices.
Why Does Index Have Two Plural Forms?
The word index has two plural forms because English adopted a word with Latin roots and developed a regular English plural alongside the traditional Latin-derived form. Both forms became established in modern usage.
The Latin Origin of Indices
The word index comes from Latin, where it referred to an indicator, sign, or person who points something out. The traditional plural indices reflects a Latin-style change in the word’s ending.
English retains similar plural patterns in several other words:
| Singular | Plural | Common context |
| Index | Indices | Mathematics and finance |
| Matrix | Matrices | Mathematics and computing |
| Appendix | Appendices | Books and formal writing |
| Vertex | Vertices | Geometry |
| Axis | Axes | Mathematics and science |
These patterns explain why indices can sound more formal or technical. The word often appears in specialized writing where traditional mathematical or scientific terminology has remained common.
For example:
- The analyst compared three economic indices over the same period.
- The equation contains several numerical indices.
- Researchers calculated the values of the relevant indices before comparing the results.
However, the Latin origin does not make indices mandatory in every sentence. Modern English allows established words to follow ordinary English plural rules, too.
Why Indexes Is Also Correct
English regularly forms plurals by adding -s or -es. The plural indexes follows this familiar pattern and has long been accepted in standard English.
This form is especially useful when discussing several reference systems, database structures, or organized lists. It also sounds natural in everyday conversation.
Consider these examples:
- The library maintains separate indexes for authors and subjects.
- The database administrator rebuilt two indexes to improve query performance.
- The researcher checked several online indexes before beginning the literature review.
The choice is not a simple contest between an old form and a wrong form. Instead, English has retained two valid plurals with overlapping meanings and different usage preferences.
Practical tip: If you are unsure, choose the form commonly used in your field. Consistency within the same document matters more than forcing one plural into every context.
When Should You Use Indices?
Use indices when writing about stock market benchmarks, mathematical quantities, or scientific measurements if that field convention favors the term. Financial journalism commonly refers to stock market indices, while mathematics uses indices in discussions of powers, sequences, and numerical positions. The word can also describe indicators or measurements used to compare data.
Indices in the Stock Market and Finance
The financial world commonly uses indices as the plural of index. A stock market index tracks the performance of a selected group of securities according to a defined methodology.
Well-known examples include:
- S&P 500: Tracks the performance of 500 leading publicly traded companies in the United States.
- Dow Jones Industrial Average: Measures the performance of 30 prominent US companies.
- NASDAQ Composite: Tracks a broad range of companies listed on the Nasdaq stock market.
Each is a single index. When discussing multiple benchmarks, financial writers often use indices.
For example:
- Several international stock indices gained value during the trading session.
- Investors compared the performance of major equity indices before adjusting their portfolios.
- The report examined inflation and consumer price indices across several economies.
The phrase stock market indices is therefore a natural choice in financial writing. However, stock market indexes is also understandable and grammatically correct. Some publications use it, so follow the terminology of your chosen publication or organization.
It is also important to distinguish an index from an investment product. An index measures a market or group of assets. An index fund attempts to track an index, while an exchange-traded fund can trade on an exchange throughout the trading day.
Example: A report might compare the S&P 500 and the NASDAQ Composite as two stock market indices. An investor might then compare funds designed to track those benchmarks.
Indices in Mathematics and Science
In mathematics, indices appears in discussions of exponents, sequences, and the positions of elements in structured data. The exact meaning depends on the subject.
For example, in the expression (2^4 = 16), the number 4 is the exponent. In British mathematical terminology, it may also be called the index or, in the plural, one of the indices in an expression.
Consider these examples:
- The student simplified the expression by applying the laws of indices.
- Researchers compared several indices to measure changes in environmental conditions.
- The algorithm uses numerical indices to identify elements in a multidimensional array.
Terminology can vary by region and discipline. American textbooks often use exponents where British textbooks may use indices. Both traditions can describe the same mathematical concept.
In computing and data science, index may also refer to a position in a sequence or an indicator associated with a measurement. In those cases, check how the particular language, library, or textbook defines the term.
Indices in Legal Writing
Legal writing does not impose one universal rule that requires indices instead of indexes. The right form depends on the jurisdiction, document type, court rules, and the style guide a legal team follows.
For example, a lawyer might prepare an index of authorities, a document index, or an index of exhibits. When several such reference lists exist, indexes is a natural plural in many legal and administrative contexts.
Examples include:
- The legal team prepared separate indexes for the exhibits submitted with each filing.
- The clerk reviewed the document indexes before locating the requested records.
- The researcher consulted several statutory indices in a historical legal collection.
The last example uses indices in a formal reference context, but that does not make it the required choice for every legal document.
If you are preparing a court filing, follow the court’s prescribed terminology and formatting requirements. In legal work, compliance with the applicable rules matters more than a general preference for either plural.
When Should You Use Indexes?
Use indexes for books, publishing, general English, and database structures when those contexts follow the regular English plural. The form is particularly common in technical documentation and everyday writing. It is also a practical default when no specialized convention or house style calls for indices.
Indexes in Books, Publishing, and Libraries
A book index is an alphabetical list of subjects, names, places, or other important terms that directs readers to relevant pages. When a book contains more than one such list, writers commonly call them indexes.
For example:
- The encyclopedia includes separate indexes for people, places, and historical events.
- The editor checked the indexes for spelling errors and missing page references.
- Researchers searched several library indexes to find relevant academic publications.
An index differs from a table of contents. A table of contents follows the order of chapters and sections, while an index usually organizes specific topics alphabetically and directs readers to the pages where they appear.
A large reference book might include:
- An index of personal names.
- A subject index.
- An index of geographical locations.
- A list of abbreviations or technical terms.
These are all natural contexts for indexes. Although indices can appear in publishing, writers do not need to use it merely because a book is formal or academic.
Indexes vs Indices in SQL and Databases
In SQL and database administration, indexes is the standard plural term for structures that help a database find records efficiently. A database index works somewhat like the index at the back of a book: it helps locate relevant information without requiring a complete search through every entry.
For example, a database might contain a customers table with thousands or millions of records. If users frequently search by email address, an index on that column can help the database locate matching records more efficiently.
Here is a basic SQL example:
CREATE INDEX idx_customers_email
ON customers (email);
This statement creates an index named idx_customers_email on the email column of the customers table.
A developer might then write:
- The database uses indexes to support common search operations.
- Adding suitable indexes can improve the performance of certain queries.
- Too many indexes can increase storage requirements and slow down data modifications.
The final point matters because indexes are not automatically beneficial in every situation. They can speed up certain reads, but they also require maintenance when data changes. Database performance depends on query patterns, index design, table size, and the database engine.
Should you write indexes or indices in SQL? Use indexes when discussing SQL database structures. It is the familiar term in mainstream database documentation and technical conversations.
Indices vs. Indexes in Python Programming
Python developers use indexes when discussing positions in lists, strings, and other sequences. However, indices may appear in mathematical computing, multidimensional arrays, and technical discussions of indexing.
Python sequences commonly use zero-based indexing. This means the first element has index 0, the second has index 1, and the third has index 2.
colors = [“red”, “blue”, “green”]
print(colors[0]) # red
print(colors[1]) # blue
print(colors[2]) # green
In this example, 0, 1, and 2 are the indexes used to access the list elements.
A developer might explain the same concept in these ways:
- The list contains three elements with indexes 0, 1, and 2.
- The function returns the indexes of matching elements.
- The algorithm calculates the indices of elements in a multidimensional array.
The third example may sound more natural in mathematical or scientific documentation. Both plural forms can appear in programming discussions, but the language or library’s established terminology should guide your choice.
For practical Python documentation, indexes is a clear default when you mean ordinary sequence positions. If you are discussing NumPy arrays, matrix operations, or mathematical indexing, indices may fit the terminology of the material you are using.
Indexes vs Indices: Quick Comparison
The table below summarizes the difference between the two plural forms across common writing situations.
| Feature | Indexes | Indices |
| Grammatical status | Correct plural | Correct plural |
| General English | Common | Accepted |
| Books and publishing | Common choice | Less common in ordinary reference contexts |
| SQL databases | Standard terminology | Less common for database structures |
| Python sequences | Common for list positions | Used in some technical contexts |
| Stock markets | Accepted | Common financial convention |
| Mathematics | Used in some contexts | Common in formal mathematical writing |
| Scientific measurements | Accepted | Common in many technical contexts |
| Legal writing | Common for document reference lists | Also possible, depending on convention |
| Singular form | Index | Index |
The main takeaway is that Indexes vs Indices is usually a question of context, not grammatical correctness. Both forms are valid, but using the form readers expect makes your writing sound more natural and precise.
Indexes or Indices in US and UK English
Both indexes and indices appear in American and British English. However, usage preferences can vary between regions, publications, and professional fields.
American English Usage
American English commonly uses indexes in general writing and technical documentation, especially when discussing book indexes or database structures.
Examples:
- The publisher updated the book’s subject indexes.
- The database administrator reviewed the existing indexes.
- The report compared several economic indices.
Notice that the final example still uses indices. American English does not reject the traditional plural. Financial and mathematical conventions can outweigh the general preference for the English-style form.
British English Usage
British English also accepts both forms. Indices is particularly familiar in mathematical writing, including phrases such as the laws of indices. Financial and scientific writing may favor it as well.
Examples:
- The students practiced the laws of indices.
- The publisher created separate indexes for the reference volumes.
- Analysts reviewed the latest market indices.
The regional distinction is a tendency, not a rigid rule. A British database administrator can naturally write indexes, while an American financial analyst can write indices.
Which form should you use in the UK? Choose the form that fits the subject and the publication’s style. For database documentation, use indexes. For mathematical or financial writing, indices is often the natural choice.
Common Mistakes When Using Indexes and Indices
Writers sometimes focus so heavily on the plural ending that they overlook the actual meaning of the sentence. Avoid these common errors.
Treating Indices as the Only Correct Plural
Incorrect claim: “Indexes is grammatically wrong because index comes from Latin.”
Better explanation: Both indexes and indices are accepted plurals in modern English. The Latin-derived form is common in several technical fields, but the regular English form is also established.
Using Indexs Instead of Indexes
Indexs is not the standard plural spelling. English words ending in x commonly take -es in this pattern.
Correct:
- One index, two indexes.
- One index, two indices.
Incorrect:
- One index, two indexs.
Assuming Every Industry Uses the Same Form
A finance writer may naturally use indices, while a database engineer may prefer indexes. Replacing one form with the other everywhere can make technical writing less consistent with the terminology readers expect.
For example, the sentence “The SQL indices improved query performance” is understandable, but “The SQL indexes improved query performance” better matches mainstream database terminology.
Switching Between Both Forms Without a Reason
Using both forms in one document is not automatically wrong. However, unnecessary switching can distract readers.
For example, if a technical report consistently discusses database indexes, do not alternate between indexes and indices simply to vary the wording. Keep the terminology consistent unless you are discussing a different field or quoting a source.
A Practical Guide to Choosing the Correct Plural
When you need to decide between indexes and indices, use this simple process.
Step 1: Identify the subject.
Are you discussing a book, a database, a stock market benchmark, a mathematical expression, or a legal reference list? The subject often determines the natural choice.
Step 2: Check established terminology.
Use indexes for SQL structures and ordinary book references. Prefer indices when it matches financial or mathematical convention.
Step 3: Consider your audience.
A general reader may find indexes more familiar. A finance professional may expect indices when reading about multiple market benchmarks.
Step 4: Follow the style guide.
If you are writing for a journal, company, court, publisher, or technical documentation team, use its preferred terminology.
Step 5: Stay consistent.
Once you select a form for a particular meaning, use it throughout the document unless the context changes.
A Real-World Editing Example
Imagine an editor reviewing a report that covers a stock market benchmark, a SQL database, and a printed research handbook.
The original draft says:
The indexes declined during the trading session. The database indices improved retrieval speed, and the handbook contains several indices.
The grammar is not necessarily wrong, but the terminology could be more consistent with common professional usage.
A clearer version would be:
The stock market indices declined during the trading session. The database indexes improved retrieval speed, and the handbook contains several indexes.
This revision aligns each plural with its typical context. The financial section uses indices, while the database and publishing sections use indexes.
The lesson is simple: good editing considers meaning and audience, not just spelling.
Frequently Asked Questions About Indexes vs Indices
Is Indices a Real Word?
Yes. Indices is a correct plural form of index. It is widely used in finance, mathematics, and scientific writing. For example, analysts discuss stock market indices, while students may study the laws of indices in mathematics.
What Is the Plural of Stock Index?
The conventional plural in financial writing is stock indices. For example, a financial report might compare the performance of several international stock indices. The phrase stock indexes is also grammatically correct, but stock indices is a common industry convention.
Can You Say Indexs?
No. Indexs is a nonstandard spelling. The two accepted plural forms are indexes and indices. Use indexes when following the regular English plural pattern, or choose indices when the context favors the traditional form.
Is Indices Singular or Plural?
Indices is plural. Its singular form is index. For example, one index can represent a market benchmark, while several indices can represent multiple benchmarks.
Should I Use Indexes or Indices in SQL and Python?
In SQL, indexes is the standard term for database structures that support data retrieval. In Python, indexes commonly describes positions in lists and other sequences. The word indices also appears in mathematical computing and discussions of multidimensional data.
How Do You Pronounce Indices?
Indices is commonly pronounced “IN-duh-seez,” with the stress on the first syllable. Its pronunciation differs from indexes, which is commonly pronounced “IN-dek-siz.” Accent differences may affect the exact vowel sounds.
Conclusion: Indexes vs Indices Made Simple
Both indexes and indices are correct plural forms of index. The right choice depends on the context, the intended audience, and the terminology established in your field.
Remember these practical distinctions:
- Use indexes for book references, general English, SQL database structures, and ordinary programming discussions.
- Use indices for stock market benchmarks and many mathematical or scientific contexts.
- In legal writing and British English, follow the conventions of the relevant document, profession, or publication.
- Use index for the singular and avoid the misspelling indexs.
