What Is the Plural of Index?
If you are writing a research paper, organizing a book, or working with data, you have probably asked yourself: What is the plural of index? The direct answer is that both indexes and indices are correct plural forms of the word index. The choice between them depends on the context, the field you are writing in, and the tone you want to use. Indexes is the standard English plural and is common in general writing and computing. Indices is the Latin plural and is preferred in academic, mathematical, and financial contexts. This guide will help you choose the right form for your writing.
Quick Answer: Indexes vs. Indices
Here is a simple breakdown to help you decide which plural to use:
- Indexes: Use this for general English, everyday conversation, and most computing contexts (e.g., database indexes, book indexes). It follows the regular English plural rule (add -es).
- Indices: Use this for formal academic writing, mathematics, statistics, finance, and scientific research. It follows the original Latin plural pattern.
Both forms are grammatically correct. Your choice signals your audience and the level of formality you intend.
Understanding the Two Plurals
The word index comes from Latin, where it meant “an informer” or “a pointer.” In Latin, the plural was indices. When English borrowed the word, it eventually developed a regular English plural, indexes, by adding the -es suffix. Today, both forms exist side by side, and each has its own preferred territory.
When to Use Indexes
Indexes is the more common plural in everyday English. You will hear it in conversation, see it in general publications, and find it in most dictionaries as the first listed plural. It is the natural choice when you are not writing for a specialized academic audience.
Common contexts for indexes:
- Book indexes (the list at the back of a book)
- Database indexes (in computing and information technology)
- Search engine indexes (how Google organizes web pages)
- General lists or catalogs
Example in a sentence: “The library has several indexes for its rare book collection.”
When to Use Indices
Indices is the preferred form in formal, technical, and academic writing. It carries a more scholarly tone and is standard in fields that use precise mathematical or statistical language. Using indices in these contexts shows that you are familiar with the conventions of the field.
Common contexts for indices:
- Mathematical indices (e.g., exponents or subscripts)
- Financial indices (e.g., stock market indices like the S&P 500)
- Statistical indices (e.g., price indices, consumer confidence indices)
- Scientific indices (e.g., refractive indices, diversity indices)
Example in a sentence: “The researcher calculated several economic indices to measure growth.”
Comparison Table: Indexes vs. Indices
| Feature | Indexes | Indices |
|---|---|---|
| Origin | English (regular plural) | Latin (original plural) |
| Formality | Neutral to informal | Formal to technical |
| Common fields | General writing, computing, libraries | Mathematics, finance, science, academia |
| Tone | Conversational, accessible | Scholarly, precise |
| Example context | “Check the indexes in the back of the book.” | “The indices show a clear trend.” |
| Reader expectation | General audience | Specialist or academic audience |
Natural Examples in Context
Seeing the words used naturally helps you understand the nuance. Here are examples from different situations:
In a conversation (informal)
“I looked at the indexes of three different books, but none of them had the information I needed.”
In an email to a colleague (neutral)
“Could you update the database indexes before the next system check? We need to make sure the search function works properly.”
In a research paper (formal)
“The refractive indices of the materials were measured at three different temperatures.”
In a financial report (formal)
“Market indices across the region showed a steady recovery in the second quarter.”
In a blog post about books (neutral)
“Good indexes are essential for nonfiction books because they help readers find specific topics quickly.”
Common Mistakes with the Plural of Index
Even experienced writers sometimes make errors with these plurals. Here are the most frequent mistakes and how to avoid them.
Mistake 1: Using “indices” in everyday conversation
While not incorrect, using indices in casual speech can sound overly formal or pretentious. If you are talking to a friend about the back of a book, indexes is the better choice.
Awkward: “I checked the indices of the cookbook, but I couldn’t find the recipe for pancakes.”
Natural: “I checked the indexes of the cookbook, but I couldn’t find the recipe for pancakes.”
Mistake 2: Using “indexes” in a formal academic paper
In a mathematics or economics paper, using indexes may mark you as unfamiliar with the field’s conventions. Professors and journal editors often expect indices.
Less appropriate: “The price indexes were calculated using a new method.”
More appropriate: “The price indices were calculated using a new method.”
Mistake 3: Mixing the two forms in the same document
Consistency is important. Choose one form and stick with it throughout your writing unless you have a clear reason to switch (for example, quoting a source that uses the other form).
Mistake 4: Thinking one form is always wrong
Some learners believe that only indices is correct because it is the Latin plural. This is not true. Indexes is fully accepted and is actually more common in many contexts. Neither form is wrong.
Better Alternatives and When to Use Them
Sometimes you may want to avoid the choice altogether. Here are alternative words that can replace index in certain contexts:
- List: Use when you mean a simple enumeration. “Please check the list of topics.”
- Catalog: Use for a systematic collection of items. “The library’s catalog is online.”
- Directory: Use in computing for a structured set of files. “The directory contains all the user files.”
- Guide: Use for a reference tool. “The guide helped me find the right section.”
- Table of contents: Use specifically for the front matter of a book. “The table of contents shows the chapter order.”
These alternatives are especially useful in informal writing or when you want to avoid any confusion about which plural to use.
Mini Practice: Test Your Understanding
Choose the correct plural form for each sentence. Answers are below.
- The book’s _____ were very detailed and helpful. (indexes / indices)
- The stock market _____ showed a sharp decline. (indexes / indices)
- Please update the database _____ after the new data is added. (indexes / indices)
- The refractive _____ of the glass were measured in the lab. (indexes / indices)
Answers:
- indexes – This is about a general book, so the regular plural is natural.
- indices – Financial markets typically use the Latin plural.
- indexes – In computing, indexes is the standard term.
- indices – Scientific and technical fields prefer indices.
Frequently Asked Questions (FAQ)
1. Is “indexes” ever wrong?
No. Indexes is a standard English plural and is correct in all contexts. It is especially common in general writing, computing, and everyday conversation. Some style guides may prefer indices for academic work, but indexes is never grammatically incorrect.
2. Is “indices” pronounced differently from “indexes”?
Yes. Indexes is pronounced IN-dek-siz (three syllables). Indices is pronounced IN-di-seez (four syllables), with the last syllable sounding like “seas.” The difference is clear when spoken aloud.
3. Which plural should I use in a business report?
It depends on the field. If your report is about financial data or economic analysis, use indices. If your report is about general business operations, such as a list of customer accounts, indexes is fine. Consider your audience and the tone of the document.
4. Can I use both plurals in the same piece of writing?
It is best to be consistent. However, if you are quoting a source that uses the other form, or if you are writing about two different fields (e.g., comparing book indexes with financial indices), you can use both. Just make sure the context makes your choice clear.
Final Thoughts
Choosing between indexes and indices is a matter of context, not correctness. For most everyday writing, indexes is the safe and natural choice. For formal academic, mathematical, or financial writing, indices shows that you understand the conventions of the field. Pay attention to your audience and the tone you want to set, and you will make the right decision every time.
If you have more questions about plural forms, explore our other guides on Common Plural Forms or visit our FAQ page for quick answers. For specific questions about singular or plural usage, check the Singular or Plural Checks section. You can also learn more about our approach on the About Us page.
