Is Appendix Singular or Plural?

The word appendix is singular. It refers to one supplementary section at the end of a book, document, or report, or to the small pouch attached to the large intestine. The plural form depends on the context: use appendices (pronounced uh-PEN-duh-seez) for academic, formal, or technical writing, and appendixes (pronounced uh-PEN-dik-siz) for general, informal, or medical contexts. Both plurals are correct, but appendices is far more common in academic and professional settings.

Quick Answer

• Singular: appendix
• Plural (formal/academic): appendices
• Plural (informal/medical): appendixes
• Rule of thumb: Use appendices for research papers, books, and legal documents. Use appendixes in everyday conversation or when talking about the body part.

Understanding the Two Plurals

English borrowed appendix from Latin, where it follows the -ix to -ices plural pattern (like index → indices). This is why appendices is the traditional, formal plural. Over time, English speakers also adopted the regular -es ending, giving us appendixes. Both forms are now standard, but they carry different tones and are used in different situations.

When to Use Appendices

Choose appendices when writing:

• Academic papers, theses, or dissertations
• Formal reports or business documents
• Legal contracts or court filings
• Books with supplementary material
• Any context where a formal, scholarly tone is expected

Example: Please refer to Appendices A and B for the full data set.

When to Use Appendixes

Choose appendixes when writing:

• Medical or anatomical descriptions (the body part)
• Casual emails or informal notes
• Everyday conversation
• General writing where a simpler plural feels natural

Example: Two patients had their appendixes removed last night.

Comparison Table: Appendices vs. Appendixes

Natural Examples

Here are examples showing how appendix and its plurals appear in real writing and speech.

Formal / Academic Context

• The report includes three appendices with survey results.
• All supplementary materials are located in Appendix C.
• Please check the appendices for the full interview transcripts.
• Each appendix must be labeled clearly in the table of contents.

Informal / Medical Context

• My cousin had his appendix out last week.
• The surgeon removed two appendixes during the operation.
• I thought the book had an appendix, but I couldn’t find it.
• She said the appendixes in the manual were confusing.

Email Context

• Dear Professor, I have attached the appendices as separate PDFs. (formal email)
• Hey, can you check the appendixes in the file I sent? (casual email to a colleague)

Common Mistakes

Even advanced learners sometimes make errors with appendix. Here are the most frequent ones and how to fix them.

Mistake 1: Using “appendices” for the body part

Incorrect: The doctor said both appendices were healthy.
Correct: The doctor said both appendixes were healthy.
Why: In medical contexts, appendixes is the standard plural for the anatomical organ. Using appendices here sounds overly formal and may confuse readers.

Mistake 2: Using “appendixes” in academic papers

Incorrect: See appendixes A and B for the methodology.
Correct: See appendices A and B for the methodology.
Why: Most style guides (APA, MLA, Chicago) prefer appendices for academic work. Using appendixes may appear less polished.

Mistake 3: Treating “appendix” as plural

Incorrect: The appendix are located at the end of the book.
Correct: The appendix is located at the end of the book. (singular) or The appendices are located at the end of the book. (plural)
Why: Appendix is singular, so it needs a singular verb.

Mistake 4: Confusing “appendix” with “index”

Incorrect: Look in the appendix for the keyword list.
Correct: Look in the index for the keyword list.
Why: An appendix contains supplementary material, while an index lists topics and page numbers. They serve different purposes.

Better Alternatives and When to Use Them

Sometimes you can avoid the plural choice altogether by rephrasing. Here are alternatives that work in specific situations.

Instead of “appendices”

• Supplementary materials – Use in formal writing when referring to the collection of extra content. Example: The supplementary materials include charts and tables.
• Additional sections – A neutral option for any context. Example: Please review the additional sections at the end.
• Back matter – Common in publishing for everything after the main text. Example: The back matter contains the glossary and appendices.

Instead of “appendixes” (medical)

• Appendix removal – Describes the procedure without pluralizing. Example: He underwent appendix removal surgery.
• Appendectomy – The medical term for surgical removal. Example: She had an appendectomy last night.

Mini Practice: Test Your Knowledge

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

1. The professor asked us to check the (appendices / appendixes) for the raw data.
2. Both patients had their (appendices / appendixes) removed successfully.
3. Please include an (appendix / appendices) with your survey questions.
4. The (appendices / appendixes) in the textbook are very detailed.

Answers

1. appendices – Academic context requires the formal plural.
2. appendixes – Medical context uses the regular plural.
3. appendix – Singular because it refers to one section.
4. appendices – Formal textbook context calls for the Latin plural.

Frequently Asked Questions

1. Is “appendix” ever used as a plural?

No. Appendix is always singular. The plural forms are appendices (formal) and appendixes (informal/medical). Never write “two appendix” or “an appendices.”

2. Which plural is more common in academic writing?

Appendices is overwhelmingly preferred in academic writing. Most style guides, including APA, MLA, and Chicago, recommend appendices for research papers, theses, and scholarly books.

3. Can I use “appendixes” in a formal email?

It depends on the tone. In a very formal email to a professor or client, use appendices. In a casual email to a coworker, appendixes is fine. When in doubt, choose appendices for professional correspondence.

4. What is the plural of “appendix” in British English?

British English follows the same rule: appendices for formal/document contexts and appendixes for medical/informal contexts. There is no difference between British and American usage for this word.

Final Tip

When you are unsure which plural to use, think about your audience and purpose. If you are writing a research paper, a book, or a formal report, choose appendices. If you are talking about the body part or writing a casual note, choose appendixes. This simple rule will keep your writing appropriate and clear in any situation.

For more help with singular and plural forms, visit our Singular or Plural Checks section. You can also explore Common Plural Forms and Confusing Plurals for related topics. If you have questions, check our FAQ page or contact us.