Index Plural Form: Rule, Examples, and Mistakes
The plural form of index is not as straightforward as adding a simple -s. In fact, index has two accepted plural forms: indexes and indices. The choice between them depends on the context, the field you are writing in, and the tone you want to set. Indexes is the standard English plural used in everyday writing, computing, and general contexts. Indices is the Latin-derived plural, preferred in mathematics, finance, and academic or formal writing. Understanding when to use each form will help you sound precise and natural in your writing.
Quick Answer: Which Plural Should You Use?
Use indexes for general English, computer databases, and book indexes. Use indices for mathematical expressions, financial markets (e.g., stock market indices), and formal academic papers. If you are unsure, indexes is almost always safe in everyday conversation and email.
Understanding the Two Plurals
Indexes (Standard English Plural)
This form follows the regular English rule of adding -es to words ending in -x. It is the most common plural in modern usage. You will see it in:
- Book publishing: “The book has three separate indexes.”
- Computing and databases: “The database rebuilds its indexes every night.”
- General lists or catalogs: “Please check the indexes at the back of the manual.”
Indices (Latin Plural)
This form comes from the original Latin plural of index. It is used in specialized, formal, or technical contexts. You will see it in:
- Mathematics: “The indices of the matrix are written as subscripts.”
- Finance and economics: “The stock market indices fell sharply today.”
- Scientific research: “The study used several health indices to measure outcomes.”
Comparison Table: Indexes vs. Indices
| Feature | Indexes | Indices |
|---|---|---|
| Origin | Standard English (add -es) | Latin plural |
| Common contexts | Books, databases, general lists | Mathematics, finance, science |
| Tone | Informal to neutral | Formal, technical, academic |
| Email/conversation | Natural and safe | Can sound overly formal |
| Example sentence | “I updated the search indexes.” | “The price indices rose by 2%.” |
Natural Examples in Context
Everyday Conversation and Email
- “Could you check the indexes in the user guide? I think we missed a few terms.”
- “The website has several indexes to help you find products faster.”
- “I rebuilt the database indexes last night, and the search is much quicker now.”
Academic and Formal Writing
- “The research paper included multiple indices of economic growth.”
- “We calculated the indices for each variable before running the regression.”
- “The consumer price indices are published quarterly by the bureau.”
Mixed Context (When Both Work)
- “The library catalog has both printed indexes and digital indices.” (Here, the writer uses indexes for the physical books and indices for the digital system, showing a subtle distinction.)
Common Mistakes with Index Plural
Mistake 1: Using “Indices” in Everyday Writing
Incorrect: “I need to update the indices in my phone’s contact list.”
Correct: “I need to update the indexes in my phone’s contact list.”
Why: Using indices for a simple phone contact list sounds unnatural and overly formal. Stick with indexes for everyday items.
Mistake 2: Using “Indexes” in Formal Academic Papers
Incorrect: “The study used several health indexes to measure patient outcomes.”
Correct: “The study used several health indices to measure patient outcomes.”
Why: In academic writing, especially in fields like medicine or economics, indices is the standard and expected form.
Mistake 3: Mixing Both Forms in the Same Document
Incorrect: “The book’s indexes are listed at the back, and the financial indices are in the appendix.”
Better: “The book’s indexes are listed at the back, and the financial indexes are in the appendix.” (If you are writing a general document, keep it consistent.)
Or: “The book’s indices are listed at the back, and the financial indices are in the appendix.” (If you are writing a formal document, use indices throughout.)
Why: Inconsistent use can confuse readers. Choose one form and stick with it unless you have a clear reason to switch.
Better Alternatives and When to Use Them
If you are unsure which plural to use, consider these alternatives:
- List – Use for simple collections: “Please check the list of terms.”
- Catalog – Use for organized collections: “The library catalog is online.”
- Directory – Use for computer file systems: “The directory contains all the files.”
- Table of contents – Use for book chapters: “The table of contents is on page iii.”
These alternatives can help you avoid the indexes vs. indices decision entirely when the context allows.
Mini Practice: Test Your Knowledge
Choose the correct plural form for each sentence. Answers are below.
- The database administrator rebuilt the (indexes / indices) overnight.
- The stock market (indexes / indices) showed a strong recovery.
- Please check the (indexes / indices) at the back of the textbook.
- The researcher calculated several health (indexes / indices) for the study.
Answers:
- indexes (computing context)
- indices (financial context)
- indexes (book context)
- indices (academic/research context)
Frequently Asked Questions
1. Is “indexes” ever wrong?
No. Indexes is never wrong in general English. It is the standard plural and is accepted in all contexts, though it may sound less formal in academic writing.
2. Can I use “indices” in an email to a colleague?
It depends on your field. If you work in finance or research, indices is normal. If you work in a general office, indexes is more natural. When in doubt, match the tone of your workplace.
3. What about the word “index” as a verb?
When index is used as a verb, its past tense is indexed, and its present participle is indexing. The plural forms discussed here only apply to the noun.
4. Are there other words like “index” with two plurals?
Yes. Common examples include appendix (appendixes / appendices), matrix (matrixes / matrices), and vertex (vertexes / vertices). The same rule applies: use the Latin plural for formal or technical contexts and the English plural for everyday use.
Final Tip for Real Writing
When you write an email, a report, or a social media post, ask yourself: Who is my reader? If you are writing to a general audience, use indexes. If you are writing to specialists or academics, use indices. This simple check will help you choose the right form every time. For more help with plural forms, visit our Plural Spelling Rules section or explore Confusing Plurals for similar tricky words.
