Plural of “appendix”

The plural of appendix is appendices (some dictionaries also accept appendixes).

Why

"Appendix" is Latin for "something attached" (from "appendere," to hang upon), and it follows the Latin -ix/-ices pattern also seen in "index/indices" and "matrix/matrices," giving "appendices" as the classical plural.

The two everyday senses of "appendix" — the small bodily organ, and the supplementary section at the back of a book or report — have genuinely split in their preferred plural: medical writing strongly favors "appendixes" for the organ (partly to sidestep confusion with the wholly unrelated "appendices" of a document), while publishing and academic writing favors "appendices" for supplementary material, a real distinction most style guides explicitly recommend rather than treat as arbitrary variation. The bodily organ's function remained genuinely uncertain in mainstream medicine for a very long time, historically dismissed as a vestigial, functionless remnant, though more recent research has proposed it may play a minor role in gut immune function and maintaining beneficial bacteria — a case where the word's underlying subject, not its grammar, has been actively revised by ongoing scientific research.

Usage note

If you're writing about the body, "appendixes" is the safer, medically preferred choice; if you're writing about the supplementary material in a report or book, "appendices" is the more expected and traditional term, and mixing the two up, while not a grave error, can read as slightly off to a specialist reader in either field.

"Appendicitis," naming the inflamed condition rather than the organ, sidesteps the plural debate entirely: like every disease named with the "-itis" suffix, it's an uncountable noun that was never going to need a plural form in the first place, regular or otherwise. Once an author picks a plural for the organ or the book section — "appendices" or "appendixes" — most academic style manuals ask for that same choice to be used consistently across an entire manuscript rather than switching partway through. Editors working on lengthy books sometimes push even further, moving bulky appendices out of the printed volume altogether into a supplementary online file, trimming the physical book while still giving readers who want the supporting material somewhere to find it.

Common real mistakes

Using "appendices" to describe multiple people's inflamed bodily organs is technically accepted by some dictionaries but reads as unusual in medical contexts, where "appendixes" is the clearly preferred and more common professional choice, reversing the more general publishing preference for the classical Latin plural. A further error some writers make is assuming the two plurals differ in formality alone, when the more accurate distinction, as discussed above, is genuinely by subject matter — medical versus publishing — rather than by how formal or casual the surrounding writing happens to be.

The wider pattern

The same Latin -ix/-ices pattern behind "appendix/appendices" also governs "index/indices" and "matrix/matrices," though "appendix" is distinctive among that group for how cleanly its two plurals have split by subject matter rather than by formality alone.

Examples

• The surgeon removed the patient's inflamed appendix.
• The report included three appendices with supporting data.
• Both appendixes were examined during the study of the rare condition.
• Readers can find the full survey questions in the appendices.
• Several appendices detailed the methodology used in each chapter.
• Doctors compared appendixes removed during similar emergency surgeries.
• The book's appendices ran nearly as long as the main text itself.
• A ruptured appendix requires immediate surgical attention.
• The thesis included five separate appendices of raw data.
• Two of the appendixes examined showed early signs of infection.
• The technical manual placed all diagrams in a separate appendix at the back.
• Doctors compared notes on appendixes removed during similar emergency procedures.
• The dissertation's appendices ran longer than several of its actual chapters.
• Reviewers asked the authors to move raw survey data into an appendix.