What’s the Plural of Index: Understanding Singular and Plural Nouns
Quick Answer
The plural of index is indexes (everyday English) or indices (math and science). Example: “This book has two indexes at the back.”
Index, Indexes, Indices — See the Pattern
Look at these sentences. Can you see what changes?
- This book has one index at the back.
- These books have separate indexes for each topic.
- The scientist used the index to find the right number.
- The scientists compared several indices in their study.
Did you see the pattern? When you talk about one, you say index. When you talk about more than one, you can say indexes or indices — but the right choice depends on the topic.
| Singular (one) | Plural — everyday | Plural — math/science |
|---|---|---|
| index | indexes | indices |
When to Use Indexes and When to Use Indices
Both indexes and indices are correct. The difference is simple:
- Indexes — use this in everyday English. Books, lists, search engines, libraries.
- Indices — use this in math, science, finance, or technical writing.
Why two forms? Because index comes from Latin. The Latin plural is indices (the ending -ex changes to -ices). But in modern English, we also just add -es to make indexes. Both are correct — it just depends on where you are using the word.
Easy way to remember: If you are writing about a book or a list, use indexes. If you are in a math or science class, use indices. When you are not sure, indexes is always safe.
The Rule in One Line: Use indexes for everyday English, indices for math and science.
Real-Life Examples With Index, Indexes, and Indices
- Can you check the index at the back of the book? (one list in a book)
- Our library has indexes for every subject. (more than one list, everyday)
- The teacher asked us to compare two indices in our math homework. (math context)
- Search engines build indexes of every website. (everyday, technology)
- The report showed several price indices going up. (finance context)
You are doing great. Now let us look at the mistakes many learners make.
Two Mistakes to Avoid With Index
Even native English speakers mix up indexes and indices sometimes — so if you find this confusing, you are not alone. There are two common mistakes:
Mistake 1: Writing “indexs”
Some learners forget that words ending in -x need -es, not just -s.
✗ Wrong: The book has two indexs.
✓ Correct: The book has two indexes.
Mistake 2: Using “indices” in everyday conversation
If you are talking about a book or a list, indexes sounds more natural. Save indices for math, science, or finance.
✗ Odd: I checked the indices of both cookbooks.
✓ Natural: I checked the indexes of both cookbooks.
How to remember: If you would say “list” instead of “index,” use indexes. If you would say “number” or “measurement,” use indices.
Other words that work the same way: appendix → appendixes/appendices, matrix → matrixes/matrices, vertex → vertexes/vertices.
Test Yourself: Index, Indexes, or Indices?
Choose the correct answer for each sentence. Click Check to see if you are right.
1. This cookbook has three helpful _______ at the back.
2. In our math class, we worked with several _______.
3. Can you look in the _______ to find the right page?
4. Stock market _______ dropped this morning.
5. Libraries use _______ to help people find books quickly.
Keep Going — You Are Building Something
You just learned the plural of index. That is one more tricky word you will never get wrong again.
But here is a question: what about matrix? Is it matrixes or matrices? It follows the same Latin pattern as index — but there is one small surprise that catches most learners off guard.
Next lesson: What’s the Plural of Matrix?
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My name is Khamis Maiouf. I am the creator of the English Teacher Site, dedicated to providing valuable resources and insights for students around the world. With a passion for education and a commitment to helping students enhance their skills, I aim to make English teaching more effective and enjoyable for both educators and students.
