Appendix Plural Form: Rule, Examples, and Mistakes

If you are writing an academic paper, a medical report, or a formal email, you might need to use the word appendix in its plural form. The direct answer is that appendix has two accepted plural forms: appendices (pronounced uh-PEN-duh-seez) and appendixes (pronounced uh-PEN-dik-siz). The choice between them depends on your field and the tone of your writing. Appendices is the standard plural in academic and formal contexts, especially in books, research papers, and legal documents. Appendixes is more common in general, informal, or medical writing. This guide explains the rule, gives clear examples, highlights common mistakes, and helps you choose the right form for your situation.

Quick Answer: Which Plural Should You Use?

Use appendices for academic, formal, or technical writing (e.g., a thesis, a textbook, a legal contract). Use appendixes for everyday conversation, informal emails, or medical contexts (e.g., a doctor talking about the human body). Both are correct, but appendices is the preferred form in most published academic work.

Why Two Plurals? The Rule Explained

The word appendix comes from Latin. In Latin, nouns ending in -ix often change to -ices in the plural. This is why appendices follows the original Latin pattern. Over time, English speakers also began adding the regular English plural ending -es, creating appendixes. Both forms are now standard, but they are used in different contexts.

When to Use Appendices (Formal and Academic)

Appendices is the go-to plural in formal writing. If you are writing a research paper, a dissertation, a business report, or a book, use appendices. It signals that you are following academic conventions. For example:

• The appendices contain the raw data and survey questions.
• Please refer to Appendices A and B for the full transcripts.

In this context, appendices sounds precise and professional. It is the form you will see in university style guides and publishing standards.

When to Use Appendixes (Informal and Medical)

Appendixes is the regular English plural. It is perfectly correct and is often used in less formal settings. In medicine, doctors and medical textbooks almost always say appendixes when referring to the organ in the human body. For example:

• The surgeon removed both inflamed appendixes.
• Some people are born with two appendixes.

In everyday conversation or a casual email to a colleague, appendixes sounds natural and avoids sounding overly academic.

Comparison Table: Appendices vs. Appendixes

Natural Examples in Context

Seeing the word used in real situations helps you remember the difference. Here are natural examples for both forms.

Examples with Appendices (Formal)

• In her dissertation, the appendices provided the complete survey instrument and consent forms.
• The editor asked the author to move the tables to the appendices at the end of the book.
• Please check the appendices for the financial statements referenced in Chapter 3.
• All supporting documents are listed in the appendices section of the report.

Examples with Appendixes (Informal/Medical)

• The anatomy textbook explained that some people have a third appendix, but two appendixes is more common.
• In his email, the researcher wrote, “I have attached the appendixes for your review.”
• The doctor noted that both appendixes appeared healthy during the scan.
• For the class project, we included two appendixes with our charts.

Common Mistakes with the Plural of Appendix

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

Mistake 1: Using “Appendices” for the Body Organ

While not grammatically wrong, using appendices to refer to the human organ sounds overly formal and unnatural in medical contexts. Stick with appendixes when talking about anatomy.

Incorrect: The patient had surgery on both appendices.
Correct: The patient had surgery on both appendixes.

Mistake 2: Using “Appendices” as a Singular Form

Some learners mistakenly treat appendices as a singular noun. Remember, appendix is the singular, and appendices is the plural.

Incorrect: Please look at the appendices on page 10. (If there is only one)
Correct: Please look at the appendix on page 10.

Mistake 3: Writing “Appendices” with an Apostrophe

Never write appendix’s or appendices’ as a plural. The apostrophe is only for possession, not for making a plural.

Incorrect: The appendix’s are at the end.
Correct: The appendices are at the end.

Mistake 4: Mixing Forms in the Same Document

Choose one plural form and use it consistently throughout your document. Switching between appendices and appendixes can confuse your reader.

Inconsistent: The appendices include data, and the appendixes include charts.
Consistent: The appendices include data and charts.

Better Alternatives and When to Use Them

Sometimes, you can avoid the plural of appendix altogether by using a clearer word. Here are some alternatives and the situations where they work better.

• Supplement – Use this for additional material that is not essential but adds value. Example: “The supplement contains extra exercises.”
• Addendum – Use this for a short addition to a document, often a correction or update. Example: “The addendum clarifies the policy.”
• Attachment – Use this for files sent with an email. Example: “Please find the attachment with the report.”
• Annex – Use this in formal or legal documents for a separate document that is referenced. Example: “The annex includes the contract terms.”

When you are writing an email to a colleague, saying “I have attached the appendixes” is fine, but “I have attached the supplements” might sound more natural if the material is optional. In a formal report, appendices remains the best choice.

Mini Practice: Test Your Understanding

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

1. The research paper includes three (appendices / appendixes) with the raw data.
2. The doctor said that both (appendices / appendixes) were normal.
3. Please refer to the (appendices / appendixes) for the full list of references.
4. In his email, he wrote, “I have attached the (appendices / appendixes) for your review.”

Answers:

1. Appendices – This is an academic context, so the formal plural is best.
2. Appendixes – This is a medical context, so the regular plural is natural.
3. Appendices – Formal instruction in a document.
4. Appendixes – An informal email is a good place for the regular plural.

Frequently Asked Questions (FAQ)

1. Is “appendices” the only correct plural?

No. Both appendices and appendixes are correct. Appendices is preferred in formal and academic writing, while appendixes is common in medical and informal contexts.

2. Can I use “appendices” in a medical journal?

It is possible, but most medical journals use appendixes when referring to the organ. If you are writing about the appendix section of a medical paper, appendices is fine. Check the journal’s style guide.

3. How do I pronounce “appendices”?

Appendices is pronounced uh-PEN-duh-seez (four syllables). Appendixes is pronounced uh-PEN-dik-siz (four syllables). The stress is on the second syllable in both forms.

4. What is the singular of “appendices”?

The singular is appendix. For example, “One appendix contains the data,” and “Two appendices contain the data.”

Final Tips for Using the Plural of Appendix

To choose the right form, think about your audience and the setting. If you are writing a university essay, a book, or a formal report, use appendices. If you are writing an email to a coworker, a medical note, or a casual blog post, appendixes is a safe and natural choice. The most important rule is to be consistent. Once you pick a form, use it throughout your entire document. For more help with tricky plurals, explore our guides on Plural Spelling Rules or check our FAQ page for common questions. If you have a specific question about your writing, feel free to contact us.