Plural of index

The Quick Answer

The plural of "index" (mathematical) is "indices." The plural of "index" (directory or catalogue) is "indexes."

The Plural of Index

In the world of mathematics, the plural of "index" is "indices." It has a Latin root, which is the derivation of the plural "indices."

When used to refer to a directory, the noun "index," follows the standard English way of marking the plural and uses -es. The plural is "indexes."
  • Indices are a useful way of more simply expressing large numbers. correct tick
  • (In mathematics, the plural of index is indices.)
  • The NASDAQ Index Directory provides a list of indexes. correct tick
  • (The plural of index (a directory) is indexes.)
The noun "index" adheres to the standard rules for forming the plurals of nouns in English (shown in the table below).

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The Standard Rules for Forming the Plurals

The table below shows the standard rules for forming the plurals of nouns in English.
Type Example of Type Forming the Plural Plural
Most Nouns lamp
scythe
add s lamps
scythes
Noun Ending s, sh, ch, x or z index
dress
add es indexes
dresses
Nouns ending [consonant] o zero
tomato
add either s or es
(There are no rules for this - you have to know.)
zeros
tomatoes
Nouns ending [vowel] o patio
ratio
add s patios
ratios
Nouns ending [consonant] y story
penny
change the y to an i and add es stories
pennies
Nouns ending [vowel] y donkey
chimney
add s donkeys
chimneys
Nouns ending f or fe dwarf
hoof
ves and/or s
(There are no rules - you have to know.)
dwarfs
hooves or hoofs
Nouns ending is index
diagnosis
change the "is" to "es" indices
diagnoses
Exceptions man
louse
some nouns undergo a vowel or letters change men
lice
More exceptions salmon
sheep
some nouns do not change at all salmon
sheep
Foreign rulings fungus
medium
some nouns adopt foreign rulings fungi
media
Read more about English spelling rules.

Why Is There Confusion over the Plural of Index?

Confusion arises because the plural of "index" change depending on the meaning of the word.
If "index" is used in a mathematical context, the plural is "indices," which has a Latin root.
When used to refer to a directory, the plural of "index" is "indexes," which adheres to the standard rules for forming the plurals of nouns in English.
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This page was written by Craig Shrives.