What Is the Plural of Thesis?

The plural of thesis is theses. This follows the rule for many English words that come from Greek, where the singular ending -is changes to -es in the plural. So one thesis becomes two theses. This is not a regular English plural like adding -s or -es; it is an irregular plural that keeps its Greek origin.

Quick Answer

Singular: thesis
Plural: theses

Use theses when you are talking about more than one thesis. For example, “She wrote three theses during her academic career.”

Why Is the Plural of Thesis Irregular?

English borrowed thesis directly from Greek. In Greek, nouns ending in -is change to -es in the plural. Other common words that follow this pattern include crisis (crises), hypothesis (hypotheses), analysis (analyses), and parenthesis (parentheses). Understanding this pattern helps you avoid common mistakes.

Comparison Table: Thesis vs. Theses

Natural Examples

Here are examples that show how thesis and theses are used in real writing, email, and conversation.

Academic Writing (Formal)

• “Each student must submit their thesis by the end of the semester.”
• “The department reviewed all theses submitted in the spring term.”
• “Her thesis argues that economic growth does not always reduce inequality.”

Email (Semi-Formal)

• “I have attached my thesis draft for your feedback.”
• “Could you send me the theses from the last conference?”
• “We are still waiting for two theses to be uploaded.”

Everyday Conversation (Informal)

• “How is your thesis going?”
• “I have to read three theses this weekend for my committee work.”
• “She finished her thesis last month and is now applying for jobs.”

Common Mistakes

Many English learners make errors with this word. Here are the most frequent ones and how to fix them.

Mistake 1: Using “thesises”

Incorrect: “He wrote two thesises for his degree.”
Correct: “He wrote two theses for his degree.”

Never add -es to thesis as if it were a regular noun. The plural is theses, not thesises.

Mistake 2: Using “thesis” for plural

Incorrect: “The professor has read all the thesis from the students.”
Correct: “The professor has read all the theses from the students.”

When you mean more than one, you must change the word to theses.

Mistake 3: Confusing “thesis” with “theses” in pronunciation

In spoken English, thesis is pronounced /ˈθiː.sɪs/ and theses is pronounced /ˈθiː.siːz/. The vowel sound in the last syllable changes from “iss” to “eez.” Practicing this difference helps you sound more natural.

Better Alternatives and When to Use Them

Sometimes you may want to avoid confusion or choose a more precise word. Here are some alternatives and their contexts.

When to use “thesis”

• When referring to one long research paper or argument.
• Example: “My thesis focuses on renewable energy policy.”

When to use “theses”

• When referring to multiple research papers or arguments.
• Example: “The library has digitized all theses from the past decade.”

Alternatives for clarity

• “Dissertation” – Often used for a doctoral thesis, especially in British English. Example: “She is writing her dissertation on medieval literature.”
• “Research paper” – A general term for an academic paper. Example: “I have to submit three research papers this semester.”
• “Argument” – When referring to the main point of a thesis. Example: “The central argument of his thesis is convincing.”

Mini Practice Section

Test your understanding with these four questions. Answers are below.

Question 1

Choose the correct sentence:
a) She has completed all her thesis for the year.
b) She has completed all her theses for the year.

Question 2

Fill in the blank: “The committee approved only one ______ this month.”

Question 3

Is this sentence correct? “The professor assigned three thesises to the class.”

Question 4

Write the plural form: “The student defended his ______ (thesis) successfully.”

Answers

Answer 1: b) She has completed all her theses for the year.

Answer 2: thesis

Answer 3: No. The correct sentence is “The professor assigned three theses to the class.”

Answer 4: theses

Frequently Asked Questions (FAQ)

1. Is “thesis” countable or uncountable?

Thesis is a countable noun. You can have one thesis or many theses. It is not uncountable like “information” or “advice.”

2. Can I use “thesis” for a short essay?

In everyday conversation, people sometimes use “thesis” loosely to mean any main argument. However, in academic contexts, a thesis is usually a long, formal research paper written for a degree. For shorter essays, use “essay” or “paper.”

3. What is the difference between “thesis” and “dissertation”?

In the United States, a thesis is often for a master’s degree, and a dissertation is for a doctoral degree. In the United Kingdom, it is often the opposite. Both words follow the same plural rule: theses and dissertations (regular plural for dissertation).

4. How do I remember the plural of thesis?

Think of other words that change -is to -es: crisis → crises, analysis → analyses, hypothesis → hypotheses. Practice these together, and the pattern will become automatic.

Final Tips for Using “Thesis” and “Theses”

• Always use theses for more than one.
• Never add a regular plural ending like -s or -es.
• Practice the pronunciation difference: /ˈθiː.sɪs/ vs. /ˈθiː.siːz/.
• In formal writing, be consistent: if you use thesis once and theses later, make sure the number matches.

For more help with similar words, visit our Common Plural Forms section. If you have questions about other tricky plurals, check our FAQ page or contact us directly. You can also read our Editorial Policy to understand how we create these guides.