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What is the plural of the word “index”?

I have a vague thought that indices would be used when index refers to a position in a sequential collection of some sort, but indexes is used when index itself refers to a collection of values.

One index in the sequence. Several indices in the sequence.

One book index. Many book indexes.

Is this correct?

  • I'd agree with your interpretation. – supercat Mar 4 '16 at 17:38

AmEng seems to prefer indexes in all contexts. BrEng is more inclined to indices, particularly in mathematics and when describing economic and social indicators. It may also be used in BrEng to describe more than one index at the back of a book, but there, as you suggest, indexes is probably more common.

  • 1
    The Society of Indexers (a British and Irish professional body) insists on indexes at the back of a book. – Tim Lymington Oct 4 '11 at 11:06

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