Indexes or Indices – The Plural Debate

Some words and terms have more than one plural form, such as “index.” The noun that means an “indicator” or “a list of specific datum” has two acceptable plurals: “indexes” and “indices.”

Want to know a little secret regarding indexes vs. indices? Keep reading to learn when to use the two words and which word is better for the UK and American writing.

Both “indexes” and “indices” are correct English plurals of the singular noun “index.” But there is a preference for indices outside North America because of solid adherence to Latin plurals.

Indexes or Indices?

Plurals of nouns take different rules. Some only need an -s or -es at the end, while others require a change in spelling. A few nouns like “index” follow both rules. 

The word “index” is a noun that means:

  • An alphabetically-arranged list of some specific datum, like subject, keyword, or author.
  • A number from several observations that serve as a measure or indicator.
  • A device that shows quantity or value.

“Index” can also be a verb that means to record specific information in a list.

In the battle of indexes versus indices, the frequency of indexes is much lower than indices. This Google Ngram Viewer shows that many writers prefer indices over indexes.

Indexes Indices

When to Use Indexes

The main difference between indices and indexes is that indexes is the Anglicization of indices. It’s wise to use “indexes” in your writing as a non-technical plural form or in an informal context. But some books that cater to formal writing settings still prefer “indexes.” 

Language experts also agree that you should use “indexes” when referring to citation listings, bibliographies, and other written documents. 

You should also use indexes over indices if you’re following the AP style guide.

The use of “indexes” also depends on the English variation. American usage and style and Canadian style guides prefer the regular plural version of “index.”

Bryan Garner wrote about the issue of indexes and indices, saying that the latter sounds pretentious. Therefore, “indexes” is always your safest choice.

The verb indexes can also be defined as the singular form, third person, present tense of “index.”

When to Use Indices

“Indices” is the irregular plural form of “index.” Irregular or foreign plurals are formed based on the language rule where they originate. For instance, “indices” has Latin roots.

Technical contexts prefer indices to indexes. The S&P Dow Jones Indices, the leading resource for investable indices, follows this form.

While “indexes” is more suitable for popular writing, the technical use of “indices” fits more in statistical contexts. You’ll rarely see non-technical writings saying “book indices” at the back. 

Merriam-Webster shows evidence that “indices” is standard in the mathematical context. It explicitly defines “indices” as numbers, expressions, or symbols connected with another to show a mathematical operation. 

There is also an advantage for indices in the UK and other areas outside North America. 

Indexes in a Sentence

Stock indexes staged a comeback Thursday afternoon as investors digested minutes from the March Federal Open Market Committee meeting, released Wednesday, showing the central bank weighing a plan to reduce its bond holdings by $95 billion per month as it tries to stamp out surging inflation. (Market Watch)

S&P Dow Jones Indices also said it was consulting with market participants on potentially removing Russian bonds from its fixed-income indexes. FTSE Russell made a similar move last week. (Wall Street Journal)

Minutes to the latest Federal Open Market Committee (FOMC) from mid-last month came out earlier this afternoon, and markets took that opportunity to drop lower across all major indexes. (Nasdaq)

Indices in a Sentence

Equity benchmark indices opened in red on Tuesday with Sensex down by 294.10 points and Nifty by 100.80 points. (The Print)

Oil majors and banks keep blue-chip index in positive territory as global indices slide. (Financial Times)

The FTSE 100 gained over 1% last week as most US, Asian and European equity indices ended the week in negative territory, weighed down by a firm hawkish outlook from the US Federal Reserve (Fed) and worries about the outcome of the first round of presidential elections in France. (IG)

Index Synonyms

  • Indicator.
  • Ratio.
  • Clue.
  • Basis.
  • Formula.
  • Guide.
  • Needle.
  • Pointer.
  • Model.
  • Indication.
  • Mark.
  • Indicia.
  • Symbol.

Indexes vs. Indices Summary

Which word wins in the battle of indexes vs. indices? The truth is that both words are standard plurals of the noun index. But here’s a better way to use them:

  • Use indexes for informal writing, AP style rule adherence, and modern American usage.
  • Use indices for technical and formal contexts and if your audience is from the UK or other countries outside North America.

Have a gander around our site for more helpful guides like this to improve your writing skills! Like this one on the usage of the idiom easy pickings.