Appendix Plural Form: Rule, Examples, and Mistakes

If you are wondering about the plural of “appendix,” the direct answer is that both appendices and appendixes are correct. The choice depends on context and audience. “Appendices” follows the Latin plural rule and is the standard choice in academic, medical, and formal writing. “Appendixes” follows the regular English plural rule (adding -es) and is acceptable in general or informal use. This guide explains the rule, gives practical examples, highlights common mistakes, and helps you choose the right form for your writing.

Quick Answer: Appendices vs. Appendixes

Use appendices for formal, academic, or technical writing (e.g., research papers, medical reports, legal documents). Use appendixes for everyday conversation, informal emails, or general writing where a simpler form feels natural. Both are grammatically correct, but “appendices” is more common in published works.

The Plural Rule for Appendix

The word “appendix” comes from Latin, where nouns ending in -ix often change to -ices in the plural. This is the same pattern as “index” becoming “indices” and “matrix” becoming “matrices.” However, English also allows regular pluralization by adding -es to words ending in -x. This gives you two acceptable forms.

When to Use Appendices

Use “appendices” in formal contexts where precision and tradition matter. This includes:

• Academic papers and theses
• Medical and scientific reports
• Legal documents
• Books and formal publications

When to Use Appendixes

Use “appendixes” in informal or general writing where simplicity is preferred. This includes:

• Business emails
• Everyday conversation
• Blog posts or casual articles
• Internal company documents

Comparison Table: Appendices vs. Appendixes

Natural Examples

Here are examples showing both forms in real contexts.

Formal Context (Appendices)

• “The research paper included three appendices with raw data and survey questions.”
• “Please refer to Appendices A and B for the full legal terms.”
• “The medical journal published the case study with supporting appendices.”

Informal Context (Appendixes)

• “I added a couple of appendixes to the report for extra details.”
• “Can you check the appendixes before we send the email?”
• “The manual has two appendixes with troubleshooting tips.”

Mixed Context

• “The textbook uses appendices for historical documents, but the teacher said appendixes is fine for our homework.”
• “In formal writing, I always use appendices, but in my notes, I write appendixes.”

Common Mistakes

Here are frequent errors learners make with the plural of “appendix.”

Mistake 1: Using “Appendices” in All Informal Writing

Some learners think “appendices” is always correct and use it in casual emails or conversations. This can sound overly formal or pretentious.

Incorrect: “I attached the appendices to the email for you.”
Better: “I attached the appendixes to the email for you.”

Mistake 2: Using “Appendixes” in Formal Academic Papers

In academic or scientific writing, “appendixes” may be seen as less professional or incorrect.

Incorrect: “The thesis includes three appendixes with survey results.”
Better: “The thesis includes three appendices with survey results.”

Mistake 3: Confusing “Appendices” with “Appendicitis”

Some learners mix up “appendices” (plural of appendix) with “appendicitis” (inflammation of the appendix). They are unrelated.

Incorrect: “The report has several appendicitis.”
Correct: “The report has several appendices.”

Mistake 4: Using “Appendices” as a Singular Noun

“Appendices” is plural. Do not use it to refer to one appendix.

Incorrect: “This appendices contains the data.”
Correct: “This appendix contains the data.”

Better Alternatives and When to Use Them

If you are unsure which form to use, consider these alternatives:

• Supplement – Use for additional material that is not essential. Example: “The supplement includes extra charts.”
• Addendum – Use for a single addition, especially in legal or formal contexts. Example: “The addendum clarifies the contract terms.”
• Attachment – Use in emails or informal documents. Example: “Please see the attachment for the details.”
• Supporting documents – Use for a group of related files. Example: “The supporting documents are in the folder.”

When to use each alternative:

• In a formal report, stick with appendices.
• In an email to a colleague, appendixes or attachment works well.
• In a legal document, addendum is more precise for a single addition.
• In a general guide, supplement is clear and neutral.

Mini Practice: Test Your Understanding

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

1. The textbook has two (appendices / appendixes) with practice exercises.
2. Please refer to the (appendices / appendixes) for the full data set.
3. I added a few (appendices / appendixes) to the email for clarity.
4. The medical journal published the study with three (appendices / appendixes).

Answers:

1. Both are acceptable, but “appendices” is more formal. In a textbook, “appendices” is common.
2. “Appendices” is better for formal reference.
3. “Appendixes” is natural for an email.
4. “Appendices” is standard in medical journals.

FAQ: Appendix Plural

1. Is “appendices” or “appendixes” more common?

In formal writing, “appendices” is more common. In everyday use, “appendixes” is gaining acceptance. Both are correct, but your audience matters.

2. Can I use “appendix” as a plural?

No. “Appendix” is singular. The plural forms are “appendices” or “appendixes.” Using “appendix” for multiple items is incorrect.

3. Does the plural change for medical vs. book appendix?

No. The same rules apply whether you are talking about the appendix in a book or the appendix in the human body. For the human body, “appendices” is standard in medical contexts, but “appendixes” is also used.

4. How do I pronounce “appendices” and “appendixes”?

“Appendices” is pronounced /əˈpɛndɪsiːz/ (uh-PEN-di-seez). “Appendixes” is pronounced /əˈpɛndɪksɪz/ (uh-PEN-dik-siz). The difference is in the last syllable.

Final Tip

When in doubt, match your choice to your context. For academic papers, formal reports, or professional publications, use appendices. For emails, casual writing, or everyday conversation, appendixes is perfectly fine. Both forms are correct, so focus on consistency and your reader’s expectations.

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