Both "indexes" and "indices" are acceptable plural forms of the word "index" or to refer to more than one index. Index is one of those rare words that have two different plurals in English. "Indices" is originally a Latin plural, while "Indexes" has taken the English way of making plurals, using –s or –es. Though both are still widely used, they take on different usage in their senses. "Indices" is used when referring to mathematical, scientific and statistical contexts.
It is used to refer to a numbers, symbols, and figures comparing a value to a standard. "Indexes" is usually used in reference to written documents, such as bibliographical or citation listings. You will likely see the words "indexes" and "indices" used interchangeably when exchanges or financial news outlets refer to more than one index, but "indices" has been more widely adopted in texts referring to stock related index.
Indexes or Indices According to Google Data
Google’s Ngram Viewer is a great tool to use for identifying the usage trends of "indices" vs. "indexes" in publications since the 1800s. The history of the usage of the word usually favors "indices" but as adoption of the Americanized plural of words increased, the word "indexes" began to appear in books and the sense of use of these plural forms became more muddled.
As much as the usage of the Americanized plural "indexes" has increased over the years, it appears to have much lower usage than "indices" around the globe. Google Trends shows that "indices" is, by far, the preferred term for Internet searches worldwide.
But if you're living in the U.S., you're not likely to raise an eyebrow using either term, as both "indices" and "indexes" have very similar levels of Internet search interest.
So Which is Correct in Terms of Grammar?
Although Bryan Garner said in the well-known grammar guide, Modern American Usage, that "indices" is pretentious and highfalutin, this technical plural form is well established and is unlikely to decrease in use any time soon.
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