The plural of appendix is appendixes or appendices.
1. Additional material added to the end of a book, document or article, typically offering supplementary statistical or bibliographic information.
2. A small tube attached to the intestines.
The word appendix has two accepted plural forms: appendixes and appendices. Being of Latin origin, the Latin plural form of appendix is appendices. Appendixes is the Anglicized version of the word, formed in the standard English way of adding -s or -es to the end of a word.
Both plural forms of appendix can be used interchangeably although the Collins dictionary states that the British English preferred form is appendices when used in its first definition to describe multiple additional materials or information.
The graph below shows the instances of both words in written English since 1900.
Singular Example: Even to this day, scientists are unsure of whether the appendix has any useful purpose in humans.
Plural Example: Appendices found in books contain all kinds of additional information such as other books on the same topic, references and citations.
The graph shows the occurances of the plural of appendix in written English since 1800 using Google's Ngram Viewer.
About 7% of Americans will develop appendicitis at some point in their life. The treatment of such a disease can involve complete surgical removal of the organ, which is a relatively safe procedure. That's a lot of appendixes removed each year!