Both are correct. Appendices is the more common plural for supplementary sections in reports, theses, and books. Appendixes is standard for the anatomical organ, and it’s also the specific spelling APA style requires — even for document sections, which breaks from the pattern most other style guides follow.
Here’s exactly which one to use, by context and by style guide, plus the practical details most articles on this topic skip.
Quick Answer
- Appendices — the more common choice for document sections in academic and formal writing, following the Latin plural pattern.
- Appendixes — standard for the anatomical body part, and required by APA style regardless of context.
- Both are listed as fully correct in every major dictionary.
- The real mistake is switching between the two within a single document — pick one and stay consistent.
Why Two Plurals Exist
Appendix comes from Latin, and Latin words ending in “-ix” traditionally form their plural by shifting to “-ices” — that’s where appendices comes from. English also has its own regular pluralization rule for words ending in “x”: add “-es,” the same pattern that turns “box” into “boxes” and “fox” into “foxes.” That regular pattern produced appendixes.
Neither form replaced the other. Both settled into standard use, which is why dictionaries list them side by side without ranking one above the other.
Which One By Style Guide
If you’re writing under a specific style guide, that guide’s preference should override any general rule of thumb:
| Style Guide | Preferred Plural |
|---|---|
| APA (7th edition) | Appendixes |
| Chicago Manual of Style | Appendices |
| MLA | Appendices |
| AP Style | Doesn’t specify; appendices is the more common convention in AP-adjacent writing |
| Medical / anatomical writing | Appendixes |
If no style guide applies to what you’re writing, appendices is the safer default for describing document sections — it’s the more widely expected form in general academic and formal writing.
Document Sections vs. the Anatomical Organ
For document sections — a thesis, report, contract, or book — appendices is the more common and expected plural in most academic and professional writing outside of APA.
For the anatomical organ, appendixes is standard, essentially without exception: “Some people are born with unusually positioned appendixes” or “the surgeon removed both appendixes” (in the rare case of a double appendix). Appendices almost never appears in clinical or anatomical contexts.
How to Label Individual Appendices
In practice, most documents label appendices with letters rather than referring to “the appendices” as a block:
- Appendix A, Appendix B, Appendix C (most common in academic and business writing)
- Appendix 1, Appendix 2, Appendix 3 (also acceptable, more common in technical and government documents)
Whichever labeling system you choose, apply it consistently, and reference it consistently in the body text (“see Appendix B for full survey data”).
Pronunciation
The two plurals aren’t just spelled differently — they’re pronounced differently:
- Appendixes: ap-PEN-dik-sez
- Appendices: ap-PEN-di-seez
Hearing the difference can help you keep the spellings straight, since the “-dik-sez” ending matches the regular English “-es” pattern, while “-di-seez” matches the Latin-derived ending.
Related Words With the Same Dual-Plural Pattern
Appendix isn’t the only Latin-origin word in English with two accepted plurals — one Latin-style, one regularized:
| Singular | Latin-Style Plural | Regular English Plural |
|---|---|---|
| Appendix | Appendices | Appendixes |
| Index | Indices | Indexes |
| Matrix | Matrices | Matrixes |
| Vortex | Vortices | Vortexes |
In each case, the Latin-style plural tends to dominate in academic, scientific, or technical writing, while the regularized plural is more common in general or everyday use — closely mirroring the appendix pattern.
Common Mistakes and Fixes
- Switching between appendixes and appendices in the same document → Pick one spelling and use it throughout, including in cross-references.
- Assuming appendices is always “more correct” → Both are standard; APA specifically requires appendixes, even for document sections.
- Writing “appendixs” → Not a real word. The two accepted plurals are appendixes and appendices only.
- Ignoring a required style guide → Check whether APA, Chicago, or another specified guide has a preference before defaulting to personal habit.
Example Sentences
- The report includes three appendices with supporting data and charts.
- Following APA guidelines, the paper’s appendixes are labeled A through C.
- The surgeon explained that some people are born with unusually positioned appendixes.
- Please refer to the appendices at the back of the manual for installation diagrams.
- Their analysis included seven figures, six appendices, and nearly 25,000 words.
Word History
Appendix entered English from Latin, where the “-ix” ending regularly shifted to “-ices” in the plural — the same pattern behind words like “index” and “matrix.” Appendixes developed later, as English speakers applied the standard English plural rule to a word that, by then, felt fully naturalized rather than foreign. Both forms have been in continuous use for centuries, which is why neither dictionaries nor most style guides treat one as an error.
Frequently Asked Questions
Is it appendixes or appendices?
Both are correct. Appendices is more common for document sections in general academic writing; appendixes is standard for the anatomical organ and is specifically required by APA style.
What is the plural of appendix in a document?
Appendices is the more common choice in most academic and formal writing outside APA. Under APA style specifically, use appendixes.
What is the plural of appendix in the body?
Appendixes. This is the standard plural for the anatomical organ in virtually all medical and clinical writing.
Does APA style use appendixes or appendices?
APA’s Publication Manual specifically calls for appendixes, even for document sections — a departure from the pattern followed by Chicago and MLA, which both prefer appendices.
Which is more formal, appendixes or appendices?
Appendices generally reads as more formal in academic writing, though appendixes is equally standard and is the required form under APA style.
Why does appendix have two plurals?
Because it comes from Latin, which pluralizes “-ix” endings as “-ices” (appendices). English also applies its own regular “-es” pluralization rule to words ending in “x,” producing appendixes. Both became standard over time.
Is “appendixs” a word?
No. That form isn’t a recognized plural. The two accepted plurals are appendixes and appendices.
Conclusion
The appendixes vs appendices choice comes down to context and style guide rather than one being universally correct. Use appendices for most formal document writing, appendixes for anatomy or when following APA style, and stay consistent within whatever document you’re writing.
