What’s the plural of “index”?
The economic indices show a positive trend.
I checked the back of the book for the relevant indexes.
The book had several useful index at the back.
We analyzed the stock market index from several countries.
The noun index accepts two plural forms, indices and indexes. Generally speaking, indices is the preferred plural when referring to the stock market index, as in ‘stock-market indices show a steady decrease’. To refer to the list of topics in a book, indexes is preferred.
Is it indices or indexes?
Index accepts both as plurals. That said, the regular plural form (indexes), which uses –es to denote a plural, is more commonly used in language today. Indices uses the original Latin suffix to show a plural, which uses –ices for singular nouns that end in -ex (e.g., appendix/appendices).
What’s an index? What’s the definition of index?
Merriam-Webster (modern day Noah Webster) defines the word index as:
A list (as of bibliographical information or citations to a body of literature) arranged usually in alphabetical order of some specified datum (such as author, subject, or keyword): such as:
a). a list of items (such as topics or names) treated in a printed work that gives for each item the page number where it may be found
b). a list of publicly traded companies and their stock prices
c). a bibliographical analysis of groups of publications that is usually published periodically. Index finger refers to your pointing finger, and is the ‘second digit’ of a pointing hand.
Irregular nouns that end in -ex/-ix and -ces/-xes
Irregular nouns that end in -ex/-ix often convert to –ces/-xes as a plural. This is a characteristic of Latin words. See the chart:
| singular | plural |
|---|---|
| index | indexes and indices |
| matrix | matrices and matrixes |
| appendix | appendices or appendexes |
| codex | codices or codexes |
| vortex | vortices or vortexes |
| vertex | vertices or vertexes |
| apex | apices or apexes |
For “index,” link “indices” to economic data like stock prices; imagine a stock chart rising with “indices” labels, while “indexes” are for books, picturing stacks of books with labeled “indexes.”.
“Index”, singular & plural, used in sentences
| “Index” (singular) used in sentences |
|---|
| A numbered index card was stationed beside each item.The invoices section index page shows a list of all invoices within the system.A death index for Newayago County can be found through the Fremont Library.She touched one with her index finger. |
| “Indices” / “indexes” (plural) used in sentences |
|---|
| A volume of elaborate indices was edited by I.As these indices represent discontinuous data, it would be preferable to use the negative binomial or the Poisson distribution.Shadows were used as indices of the sun’s position, in combination with angular divisions.In the same way, by dividing by powers of so we may get negative indices. |
Index synonyms
- catalogue
- list
- record
- file
- schedule
Origin of the word index
From etymonline on index:
“The forefinger,” from Latin index (genitive indicis) “one who points out, discloser, discoverer, informer; forefinger (because used in pointing); pointer, sign; title, inscription, list,” literally “anything which points out,” from indicare “to point out,” from in- “into, in, on, upon”.
Read on other topics in grammar
Sources
- Definition of index finger.
- Definition of index.
- Synonyms for index.
- Index sentence examples.
- Origin on index.
Work Sheet
According to the blog post, which of the following is NOT a correct plural form of “index”?
When referring to economic data or stock market trends, which plural of “index” is generally preferred?
Which plural form of “index” is preferred when referring to the list of topics or names found at the back of a book?
The plural form “indices” uses a suffix that originates from which language?
According to the post, using the singular “index” when referring to multiple items is incorrect. Which sentence below demonstrates this common mistake?
The latest economic showed a positive trend.
She consulted the back of the book, which contained several useful .
Instead of saying “several useful index”, the correct plural form for a book list is .
When referring to the stock market trend for just one country, you would use the singular word .
We analyzed the cost of living from several different cities.
Frequently Asked Questions
What is the plural of index?
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When do I use ‘indices’?
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When do I use ‘indexes’?
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Are both “indices” and “indexes” correct?
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Which plural is more common now?
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Yash, D. "What’s the Plural of Index?." Grammarflex, Jun 3, 2025, https://www.grammarflex.com/whats-the-plural-of-index/.
