If you have ever written a research paper, compiled a book, or worked with data, you have likely needed the plural of index. The direct answer is that both indexes and indices are correct plurals, but they are used in different contexts. Indexes is the standard plural for everyday use, such as in books or databases. Indices is the technical plural preferred in mathematics, finance, and scientific writing. Choosing the wrong one can make your writing sound informal in an academic paper or overly technical in a general email.
Quick Answer: Which Plural Should You Use?
- Indexes – Use for general writing, book indexes, database indexes, and everyday conversation. Example: “The library has multiple indexes for different subjects.”
- Indices – Use for formal academic writing, mathematics, economics, and scientific contexts. Example: “The economic indices showed a steady recovery.”
If you are unsure, indexes is almost always safe for non-technical writing. For academic papers, check your style guide or field convention.
Understanding the Two Plurals
The word index comes from Latin, which is why it has two plural forms. English has kept the Latin plural indices for specialized fields, while the regular English plural indexes has become common in everyday use. This is similar to words like appendix (appendices/appendixes) and matrix (matrices/matrixes).
When to Use “Indexes”
Indexes is the standard plural in modern English. You will see it in:
- General writing and conversation
- Library catalogs and book publishing
- Database and computer science contexts (e.g., “database indexes”)
- Informal emails and reports
Example: “The book has three separate indexes: one for names, one for places, and one for subjects.”
When to Use “Indices”
Indices is the preferred form in technical and academic fields. You will see it in:
- Mathematics and statistics (e.g., “price indices”)
- Economics and finance (e.g., “stock market indices”)
- Scientific research papers
- Formal academic writing
Example: “The researchers calculated several health indices to measure population well-being.”
Comparison Table: Indexes vs. Indices
| Aspect | Indexes | Indices |
|---|---|---|
| Origin | Regular English plural | Latin plural |
| Formality | Informal to neutral | Formal to technical |
| Common contexts | Books, databases, everyday writing | Mathematics, economics, science |
| Email tone | Natural and conversational | Formal and precise |
| Academic use | Acceptable in some fields | Preferred in most academic writing |
| Example sentence | “Please check the indexes at the back of the report.” | “The consumer price indices rose by 2%.” |
Natural Examples in Context
In an Email (Informal)
“Hi Mark, I updated the database indexes this morning. The search should be faster now. Let me know if you notice any issues.”
In a Research Paper (Formal)
“The body mass indices of the participants were recorded at baseline and after six months. Statistical analysis revealed significant changes in these indices over time.”
In a Conversation
“I love how this textbook has multiple indexes. It makes finding topics so much easier.”
In a Financial Report
“The market indices showed mixed results, with the S&P 500 and Dow Jones indices moving in opposite directions.”
Common Mistakes with the Plural of Index
Mistake 1: Using “Indices” in Casual Writing
Incorrect: “I checked the indices at the back of the cookbook.”
Correct: “I checked the indexes at the back of the cookbook.”
Why: In everyday contexts, indexes sounds more natural. Using indices here feels overly formal and out of place.
Mistake 2: Using “Indexes” in Academic Papers
Incorrect: “The study used several economic indexes to measure growth.”
Correct: “The study used several economic indices to measure growth.”
Why: Most academic journals and style guides prefer indices for technical terms. Check your field’s convention.
Mistake 3: Mixing Both Forms in the Same Document
Incorrect: “The book indexes were helpful, but the price indices were confusing.”
Better: Choose one form and stick with it unless you have a clear reason to switch. If you are writing a general document, use indexes throughout. If it is technical, use indices.
Mistake 4: Using “Indices” as a Singular
Incorrect: “This indices shows the trend.”
Correct: “This index shows the trend.”
Why: Indices is always plural. The singular form is always index.
Better Alternatives and When to Use Them
Sometimes, you can avoid the confusion entirely by using a different word. Consider these alternatives:
- List – Use for simple collections. Example: “Please see the list of topics at the end.”
- Catalog – Use for organized collections. Example: “The library catalog is online.”
- Guide – Use for reference materials. Example: “The study guide includes a helpful index.”
- Measure – Use for statistical data. Example: “The economic measures showed improvement.”
- Indicator – Use for technical data. Example: “The health indicators were positive.”
These alternatives can make your writing clearer and help you avoid the indexes/indices dilemma entirely.
Mini Practice: Test Your Knowledge
Choose the correct plural form for each sentence. Answers are below.
- The book has two (indexes / indices) at the back.
- The stock market (indexes / indices) fell sharply yesterday.
- Please update the database (indexes / indices) after the migration.
- The researcher calculated several body mass (indexes / indices) for the study.
Answers:
- Indexes – General book reference.
- Indices – Financial context, formal.
- Indexes – Database context, technical but standard.
- Indices – Scientific research, formal.
Frequently Asked Questions
1. Is it ever wrong to use “indexes”?
No, indexes is never grammatically wrong. However, in very formal academic or scientific writing, some readers may expect indices. If you are submitting a paper to a journal, check their style guide.
2. Can I use “indexes” in a math paper?
It depends on the journal. Many mathematics publications prefer indices because it is the traditional Latin plural. However, some modern math textbooks use indexes. When in doubt, look at recent papers in your target journal.
3. What about “index” as a verb? Does it change the plural?
No. When index is used as a verb (e.g., “The database indexes records quickly”), the verb form does not affect the noun plural. You still use indexes or indices for the noun.
4. Are there other words like “index” with two plurals?
Yes. Common examples include appendix (appendices/appendixes), matrix (matrices/matrixes), and vertex (vertices/vertexes). The same rule applies: use the Latin plural for technical contexts and the English plural for everyday writing.
Final Tips for Choosing the Right Plural
- If you are writing an email to a colleague, use indexes.
- If you are writing a research paper in economics, use indices.
- If you are writing a book, use indexes for the back-of-book reference.
- If you are writing about databases, indexes is standard in computer science.
- If you are writing about financial markets, indices is preferred.
Remember, consistency is key. Once you choose a form, use it throughout your document. If you are still unsure, indexes is the safer choice for most non-technical writing, while indices shows attention to academic convention.
For more help with confusing plurals, visit our Confusing Plurals section. You can also explore Common Plural Forms or check our FAQ for additional questions. If you have specific concerns, feel free to contact us.
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