"What's the plural of index?"

The plural of index is indexes or indices.

Definition of index:

1. A list, catalogue or directory ordered alphabetically or numerically. Examples include lists of citations at the end of an article or book and lists of publicly traded stocks.

2. In mathematics, an index number can refer to a number or expression which is raised to a power. Index numbers are also known as exponents.

Indexes or Indices?

The plural for index can be formed in two different ways depending on the definition of the word in context.

When used to refer to multiple lists or directories, the word indexes is used. This follows the regular English rule for pluralization by adding the suffix -es to the end of the word.

When used is a mathematical sense, the irregular plural of indices is preferred.

The rules however, are not completely clear-cut and the words can be used interchangeably, especially when used informally.

American informal English tends to be prefer the word indexes whilst British English uses indices.

In formal, academic and financial contexts, both prefer the use of the word indices.

Plural of Index Example Sentences

Here are some examples to illustrate the difference between the singular form "index", the plural form "indexes", and the alternative plural form "indices":


  • "The index of the report included a list of key findings and recommendations." (This sentence is talking about one index.)

Plural (indexes):

  • "The indexes in the research database allowed me to easily compare data from multiple sources." (This sentence is talking about a group of indexes.)
  • "I searched through the indexes of the library's catalog to find the books I needed." (This sentence is talking about a group of indexes.)

Plural (indices):

  • "The economic indices showed signs of improvement in the housing market." (This sentence is talking about a group of indices.)
  • "We used the indices of the matrix to determine the position of each element." (In this sentence, "indices" refers to the numerical labels that are used to identify the rows and columns of a matrix.)

Indexes vs. indices

The graph shows the occurances of the plural of index in written English since 1800 using Google's Ngram Viewer.



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The plural of index is indexes or indices
The plural of index is indexes or indices