Despite widespread notions that the label of "incorrect" can be applied to either appendixes or appendices in any sense of the word (such as book appendixes or anatomic appendixes), major dictionaries and usage guides show that either plural form is acceptable, and that although the naturalized English regular form, appendixes, is more often prescribed to be preferable, the English irregular form (that is, the Latin regular as loanword), appendices, appears more often in published books; and no basis is given for any of the prescriptions, which are widely contradicted anyway. In short, unlike with many prescriptive-descriptive differences where one option can objectively be said to have a wide consensus of being preferable among careful or defensive writers although the other is not incorrect, in this instance there is very little basis for even asserting any consensus preference at all, given the collation that follows. What can be said with certainty is all of the following: some people use the Latin inflection because they think it is necessary; some other people use the Latin inflection because they don't want to appear ignorant to anyone who holds that misapprehension; some confidently use the English regular inflection because they know it is not wrong and is even preferred by "some people"; many people aren't sure whether one is better than the other, but they feel some degree of need to find out, so as not to appear ignorant to any of their readers; many other people both aren't sure and don't much care anyway; and readers of this entry should come away understanding that the last-mentioned group's approach is validated.
"Both are correct plural forms for appendix, but appendixes is preferable outside scientific contexts […] Nevertheless, the nonnative plural is more frequent [per Google Books Ngram data] by a 3-to-1 margin."
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