Q From Bert Forage, Australia; related questions came from Mark Smith and David Parks: I wish you wouldn’t spell the plural of index as indexes in your side banner! We’re being dragged screaming into the American version of English.
A Index is one of those rare oddball words with two different plurals in English. English copied the original Latin plural at first, making indices. As with a lot of other Latin plurals, the standard English way of marking the plural, using -s or -es, has progressively been taking over, making indexes.
This is why we now prefer crematoriums to the Latin crematoria and forums to fora, though the Latin plurals are still regularly used by some writers. Other words retain their Latin plurals, but we have to work at remembering them because the English plural marker has otherwise so few exceptions: apices (of apex), corpora (of corpus), helices (of helix), matrices (of matrix), vertices (of vertex) and many others.
Index is a good example of a small subgroup in which both plurals are alive and well but in which usage has separated their senses. Another is appendix, in which appendices refers to books but appendixes to bodily organs.
Indices has survived in scientific work, especially in mathematics. When index refers to a number or symbol, such as an exponent — the superscript figure 2 to indicate a number is to be squared, for example, as in x2 — it has the Latin plural. Statisticians also talk about indices when they mean figures comparing a value to a standard, so that retail prices index turns into retail prices indices in the plural. An example appeared in the Observer on 12 October 2008: “Britain has more national house price indices than any other country — and they have seldom varied so much in their analysis.” Despite Bryan Garner’s comment in his Modern American Usage that indices is pretentious and highfalutin, this technical plural form is well established and unlikely to fall out of use any time soon.
But it’s the only situation in which it’s found. The usual plural is indexes, which first appeared in the seventeenth century. If you’re talking about several of the sort in books, for example, that’s the right one to use. Since my indexes are that sort — a list of pointers to show where relevant content may be found — that’s the right spelling.
By the way, people sometimes think indices is an English plural and so make a singular noun indice from it (apice, matrice and vertice are also occasionally seen, created in the same way). Examples of indice can be found going back a century or more and not always in uneducated writing by any means. Charles Doyle contributed one in a note that appeared in the Winter 1979 issue of American Speech: “At a recent academic gathering, a literary savant began his speech with a quotation that spoke of certain indices. Thereafter, at least a dozen times, the speaker referred to this or that indice (ending like jaundice).” It also appeared in the Washington Post on 22 August 2008: “Yet as an indice of some of the lines of attack that the McCain camp is employing it is of great interest.” Thus does language change ...