Common Mistakes with the Plural of Index

The most common mistake with the plural of “index” is using “indices” in casual conversation or “indexes” in formal academic writing. The correct plural depends entirely on context: use indexes for everyday references (like book indexes or database indexes) and indices for technical, mathematical, or financial contexts. Both are correct, but using the wrong one can sound unnatural or overly formal.

Quick Answer

• Indexes: Use for general, non-technical contexts (book indexes, website indexes, list indexes).
• Indices: Use for technical, scientific, mathematical, or financial contexts (stock market indices, statistical indices, scientific indices).
• Both are accepted in modern English, but indexes is more common in everyday writing and conversation.

Understanding the Two Plurals

The word “index” comes from Latin, which is why it has two plural forms. English has kept the original Latin plural “indices” for technical use, while also developing the regular English plural “indexes” for general use. This dual system is similar to other Latin-derived words like “appendix” (appendices/appendixes) and “matrix” (matrices/matrixes).

When to Use “Indexes”

“Indexes” is the standard English plural and works best in:

• Everyday conversation
• Business emails and reports
• General writing about books, documents, or databases
• Informal contexts

Examples:

• “The book has three separate indexes for names, places, and topics.”
• “I need to update the database indexes after adding new records.”
• “Please check the indexes at the back of each manual.”

When to Use “Indices”

“Indices” is the Latin plural and is preferred in:

• Academic writing (especially mathematics, science, and economics)
• Financial reports and stock market analysis
• Technical documentation
• Formal research papers

Examples:

• “The stock market indices showed a significant drop this quarter.”
• “The researcher calculated several statistical indices for the study.”
• “In mathematics, we use indices to represent exponents.”

Comparison Table

Natural Examples

In Conversation

“I looked through the indexes of three different books, but I couldn’t find the information I needed.”

In Email

“Dear Team, please verify that all database indexes are properly maintained before the system update.”

In Academic Writing

“The study examined various health indices including body mass index and blood pressure readings.”

In Financial Context

“The market indices have been volatile due to recent economic policy changes.”

Common Mistakes

Mistake 1: Using “Indices” in Casual Conversation

Incorrect: “I checked the book’s indices but couldn’t find the chapter.”
Correct: “I checked the book’s indexes but couldn’t find the chapter.”
Why: In everyday conversation, “indexes” sounds more natural and less pretentious.

Mistake 2: Using “Indexes” in Technical Writing

Incorrect: “The economic indexes were recalculated using new data.”
Correct: “The economic indices were recalculated using new data.”
Why: In formal economic or scientific writing, “indices” is the standard term.

Mistake 3: Mixing Both in the Same Document

Incorrect: “The report includes several indexes and statistical indices.”
Correct: “The report includes several statistical indices.” (Choose one style and stick with it)
Why: Consistency is important in formal writing.

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

When You’re Unsure

If you’re not sure which plural to use, indexes is almost always safe in general writing. For academic or technical work, check the style guide of your field or publication.

Alternative Phrasing

Sometimes you can avoid the issue entirely:

• Instead of “book indexes,” say “the index section of the book.”
• Instead of “market indices,” say “stock market indicators.”
• Instead of “database indexes,” say “database search tools.”

Context-Specific Recommendations

• For business emails: Use “indexes” unless writing about financial markets.
• For academic papers: Use “indices” in mathematics, science, and economics.
• For general blog posts: Use “indexes” for readability.
• For technical documentation: Follow your company’s style guide.

Mini Practice Section

Choose the correct plural form for each sentence.

Question 1: The library has several (indexes/indices) for different subject areas.
Answer: indexes (general context about library books)

Question 2: The economic (indexes/indices) were published by the central bank.
Answer: indices (formal financial context)

Question 3: Please update the database (indexes/indices) after importing new records.
Answer: indexes (technical but general database context)

Question 4: The study calculated several health (indexes/indices) for the population.
Answer: indices (scientific research context)

FAQ

1. Is “indexes” ever wrong?

No, “indexes” is never wrong in general English. It is the standard plural for most contexts. The only time it might be considered incorrect is in very formal academic or technical writing where “indices” is expected.

2. Can I use “indices” in everyday conversation?

You can, but it may sound overly formal or academic. In casual conversation, “indexes” is more natural. For example, saying “I checked the book’s indices” might make you sound like you’re trying too hard.

3. Which plural is more common in American English?

In American English, “indexes” is more common in general writing, while “indices” is reserved for technical fields. British English follows a similar pattern, though “indices” may appear slightly more often in formal British writing.

4. What about the word “index” in computer programming?

In programming, both forms are used. “Indexes” is common for database indexes, while “indices” is often used in mathematical or algorithmic contexts. Many style guides for technical documentation prefer “indexes” for clarity.

For more help with confusing plurals, visit our Confusing Plurals section. You can also check our Common Plural Forms or Plural Spelling Rules for additional guidance. If you have questions, see our FAQ or contact us.