What Is the Plural of Index?
The word index has two correct plural forms: indexes and indices. Which one you use depends on the context. Indexes is the standard English plural, common in everyday writing and general use. Indices is the Latin-derived plural, preferred in technical, academic, and formal contexts, especially in mathematics, finance, and science. Both are correct, but they are not always interchangeable.
Quick Answer
- Indexes – Use for general references, lists, or tables of contents. This is the simpler, more common choice.
- Indices – Use for technical or formal contexts, such as stock market indices, mathematical indices, or scientific indices.
Understanding the Two Plurals
The reason index has two plurals comes from its history. English borrowed the word from Latin, where the plural was indices. Over time, English speakers also formed a regular plural by adding -es, giving us indexes. Both forms are now standard, but they have settled into different areas of use.
When to Use Indexes
Indexes is the plural you will see most often in everyday English. It is the natural choice when talking about:
- Lists of topics in books or documents
- Database indexes
- Search engine indexes
- General references or catalogs
In informal writing, emails, and conversation, indexes sounds natural and is rarely questioned. It follows the regular English plural pattern, so it feels familiar to most readers.
When to Use Indices
Indices is the preferred form in specialized fields. You will encounter it in:
- Finance: stock market indices (e.g., the S&P 500, Dow Jones indices)
- Mathematics: exponents or subscript numbers (e.g., the indices in a matrix)
- Science: measurement indices (e.g., body mass index, refractive indices)
- Academic writing: formal research papers and technical reports
Using indices in these contexts signals that you are familiar with the terminology of the field. In everyday conversation, however, it can sound overly formal or technical.
Comparison Table: Indexes vs. Indices
| Aspect | Indexes | Indices |
|---|---|---|
| Origin | Regular English plural | Latin plural |
| Common contexts | Books, databases, search engines, general lists | Finance, mathematics, science, formal writing |
| Tone | Neutral, everyday, informal to formal | Formal, technical, academic |
| Email use | Natural for most business and personal emails | Appropriate for technical or financial reports |
| Conversation use | Common and comfortable | Rare; may sound stiff |
| Example sentence | “The book has two indexes at the back.” | “The economic indices showed a decline.” |
Natural Examples
Seeing both forms in real sentences helps you choose the right one.
Examples with Indexes
- “I checked the library’s indexes to find the article.”
- “The website’s search indexes need to be updated.”
- “We created separate indexes for each chapter.”
- “The database indexes improved query speed.”
Examples with Indices
- “The stock market indices rose sharply today.”
- “In mathematics, we use indices to show powers.”
- “The refractive indices of the materials were measured.”
- “Several health indices were used in the study.”
Common Mistakes
Even advanced learners sometimes mix these up. Here are the most frequent errors.
- Using indices in everyday conversation: Saying “I looked at the indices in the back of the book” sounds unnatural. Use indexes instead.
- Using indexes in technical writing: In a finance report, writing “the stock market indexes” may mark you as a non-specialist. Use indices.
- Treating them as interchangeable: While both are correct, they are not always interchangeable. Context matters. A math teacher will expect indices; a librarian will expect indexes.
- Forgetting the spelling: Indices ends with -ices, not -eces or -ises. A common misspelling is indeces.
Better Alternatives and When to Use Them
Sometimes you may want to avoid the choice altogether. Here are alternatives that work in many situations.
- List – “The list of topics is at the end.” Works for general references.
- Catalog – “The catalog of terms is searchable.” Good for databases or collections.
- Table of contents – “The table of contents shows the chapters.” Specific to books.
- Measurements – “The measurements were recorded.” Useful in scientific contexts.
- Indicators – “The economic indicators improved.” A clear alternative for financial or statistical contexts.
Use these alternatives when you want to avoid confusion or when the exact plural form feels awkward. For example, in an email to a general audience, “the list of stock market indicators” is clearer than “the stock market indices.”
Mini Practice: Test Your Understanding
Choose the correct plural form for each sentence. Answers are below.
- The book has three (indexes / indices) for different subjects.
- The financial (indexes / indices) were updated quarterly.
- We need to rebuild the search (indexes / indices) on the website.
- The mathematical (indexes / indices) in the formula are confusing.
Answers
- Indexes – General book references use the everyday plural.
- Indices – Financial contexts prefer the technical plural.
- Indexes – Search engine and database contexts use the regular plural.
- Indices – Mathematics uses the Latin plural.
FAQ: Common Questions About the Plural of Index
1. Is it ever wrong to use indexes?
No. Indexes is always correct in general English. The only time it might look out of place is in highly technical or formal academic writing where indices is the expected form. In most emails, conversations, and business writing, indexes is perfectly fine.
2. Can I use indices in a casual email?
You can, but it may sound overly formal. If you are writing to a colleague about a book’s back matter, indexes is more natural. If you are discussing financial data, indices is appropriate even in email.
3. What about the word index as a verb?
When index is used as a verb, its past tense is indexed, and the present participle is indexing. The plural forms indexes and indices only apply to the noun.
4. Are there other words like index with two plurals?
Yes. Several English words borrowed from Latin have two plural forms. Examples include appendix (appendixes / appendices), matrix (matrixes / matrices), and vertex (vertexes / vertices). The pattern is similar: the regular English plural is for general use, and the Latin plural is for technical contexts.
Final Note
Choosing between indexes and indices is about matching your audience and context. For everyday writing, indexes is safe and clear. For technical or formal work, indices shows precision. Both are correct, so the key is knowing when each one fits best.
If you have more questions about plural forms, visit our Common Plural Forms section. For help deciding between singular and plural, check our Singular or Plural Checks page. You can also read our FAQ for quick answers. For any feedback, see our Contact Us page or review our Editorial Policy.
