What Is the Plural of Appendix?

The word appendix has two standard plurals: appendices and appendixes. Both are correct, but they are used in different contexts. Appendices is the traditional Latin plural and is preferred in academic, medical, and formal writing. Appendixes is the English plural and is common in general, informal, and everyday use. In medical contexts, both forms appear, but appendices is more frequent when referring to the anatomical organ, while appendixes is often used for supplementary material in documents.

Quick Answer

• Appendices (Latin plural) – preferred in formal, academic, and medical writing.
• Appendixes (English plural) – acceptable in general and informal contexts.
• Both are correct. Choose based on your audience and tone.
• In medical writing, appendices is more common for the organ; appendixes is more common for document supplements.

Understanding the Two Plurals

The reason for two plurals comes from the word’s origin. Appendix is a Latin noun that belongs to a group of words ending in -ix or -ex that change to -ices in the plural. This is the same pattern you see in index becoming indices or matrix becoming matrices. Over time, English speakers began adding the regular English -es ending to these words, creating appendixes. Today, both forms are widely accepted, but they carry different stylistic weight.

When to Use Appendices

Use appendices when you are writing for a formal, academic, or medical audience. This form signals that you are following traditional conventions. It is the standard choice in:

• Medical journals and textbooks
• Research papers and dissertations
• Legal documents
• Formal reports
• Clinical notes and patient records

Example: The surgeon removed both inflamed appendices during the procedure.

When to Use Appendixes

Use appendixes in everyday writing, casual conversation, and when referring to supplementary material in a book or document. It sounds more natural to many English speakers and avoids the slightly formal tone of the Latin plural. It is common in:

• Business emails
• Blog posts and articles
• Informal reports
• Everyday conversation

Example: Please check the appendixes at the back of the manual for the full list of references.

Comparison Table: Appendices vs. Appendixes

Natural Examples

Here are examples that show how each plural is used in real situations. Notice how the context and tone change.

• Medical context (formal): The pathologist noted that both appendices showed signs of chronic inflammation.
• Medical context (conversation): The doctor said they had to take out both appendixes during the surgery.
• Academic writing: The full data sets are available in the appendices at the end of the thesis.
• Business email: I have attached the appendixes to this email for your review.
• Everyday conversation: I never read the appendixes in textbooks; they are too boring.
• Patient education: Some people are born with two appendices, though it is rare.

Common Mistakes

Even advanced English learners sometimes make errors with this word. Here are the most frequent mistakes and how to avoid them.

• Mistake: Using appendix’s as a plural.
  Correct: Appendices or appendixes. The apostrophe is only for possession, not plural.
• Mistake: Thinking appendices is always wrong.
  Correct: It is the standard form in formal and medical writing.
• Mistake: Using appendixes in a formal research paper.
  Correct: Most academic style guides prefer appendices.
• Mistake: Mixing both forms in the same document.
  Correct: Choose one and use it consistently throughout.
• Mistake: Pronouncing appendices as “ap-pen-dye-sees” instead of “ap-pen-di-sees”.
  Correct: The c is soft, like an s sound.

Better Alternatives and When to Use Them

Sometimes you may want to avoid the confusion altogether. Here are some alternatives depending on what you mean.

• For the anatomical organ: Use vermiform appendix (singular) or appendices (plural). In patient communication, you can say appendix for singular and clarify the plural if needed.
• For document supplements: Use supplementary materials, addenda, or supporting documents. These are clear and avoid the plural issue entirely.
• In informal writing: Appendixes is perfectly fine. Do not feel pressured to use appendices if it sounds unnatural to you.
• In formal writing: Stick with appendices. It is the expected form and shows attention to convention.

Mini Practice: Test Your Understanding

Choose the correct plural form for each sentence. Answers are below.

1. The surgeon removed both __________ during the operation. (appendices / appendixes)
2. Please refer to the __________ at the end of the report for the raw data. (appendices / appendixes)
3. In her email, she mentioned that the __________ were missing from the proposal. (appendices / appendixes)
4. The medical textbook included detailed __________ on rare conditions. (appendices / appendixes)

Answers

1. Appendices (medical, formal context)
2. Appendices (formal report, academic tone)
3. Appendixes (informal email, general context)
4. Appendices (medical textbook, formal writing)

Note: In questions 1 and 4, appendixes would also be understood, but appendices is more appropriate for the context. In question 3, appendices would sound overly formal for an email.

Frequently Asked Questions

Is “appendices” the only correct plural?

No. Both appendices and appendixes are correct. Appendices is the traditional Latin plural and is preferred in formal and medical writing. Appendixes is the English plural and is common in general use.

Which plural should I use in a medical journal?

Use appendices. Medical journals and textbooks almost always follow the Latin plural convention. It is the standard in professional medical writing.

Can I use “appendix” as a plural?

No. Appendix is singular. Using it as a plural is a common error. Always use appendices or appendixes for more than one.

What is the plural of “appendix” in a book?

Both are used. In formal publishing, appendices is more common. In informal or self-published works, appendixes is also widely accepted. Check the style guide of the publisher if you are unsure.

Final Note

Choosing between appendices and appendixes is a matter of context and audience. If you are writing a formal medical report, a research paper, or a legal document, use appendices. If you are writing an email, a blog post, or having a casual conversation, appendixes is perfectly fine. The most important rule is to be consistent. Pick one form and use it throughout your document. For more help with medical plurals, explore our guides on Common Plural Forms or check our FAQ for quick answers to other questions.