Thesis Plural Form: Rule, Examples, and Mistakes
The plural of thesis is theses. This is not a regular English plural; it follows the Greek pluralization pattern where the singular ending -is changes to -es. So one thesis, two theses. This rule applies to a small group of academic and scientific words borrowed from Greek, and getting it wrong is one of the most common spelling errors in formal writing.
Quick Answer: Thesis vs. Theses
If you need the plural form of thesis right now, here is the direct answer:
- Singular: thesis
- Plural: theses
Do not write thesises, thesis’s, or thesi. The correct plural is always theses, pronounced THEE-seez.
Why the Plural Is “Theses” and Not “Thesises”
English has many words that come from Greek, and some of them keep their original plural forms. Thesis is one of these. In Greek, nouns ending in -is often change to -es in the plural. Other common examples include:
- crisis → crises
- analysis → analyses
- hypothesis → hypotheses
- parenthesis → parentheses
- emphasis → emphases
Once you learn this pattern, you can apply it to any word in this group. The mistake usually happens because learners try to add a regular English -s or -es ending, which produces the incorrect thesises.
Comparison Table: Thesis vs. Theses
| Feature | Thesis (Singular) | Theses (Plural) |
|---|---|---|
| Number | One | Two or more |
| Pronunciation | THEE-sis | THEE-seez |
| Spelling pattern | Ends in -is | Ends in -es |
| Common error | None (singular is easy) | Writing thesises or thesis’s |
| Grammar agreement | Use singular verb: This thesis is… | Use plural verb: These theses are… |
| Example sentence | Her thesis was accepted. | Both theses were rejected. |
Natural Examples in Context
Seeing the word used in real situations helps you remember the correct form. Here are examples from different settings.
Academic Writing (Formal)
- Each student must submit one thesis by the deadline.
- The library holds over two hundred theses from the last decade.
- Her thesis on climate change won the department award.
- We compared the methodologies used in both theses.
Everyday Conversation (Informal)
- I’m still working on my thesis. It’s taking forever.
- How many theses did your professor supervise this year?
- He keeps talking about his thesis at dinner. It’s all he thinks about.
- They finished their theses last month and finally graduated.
Email Context
- Formal email: Please find attached my thesis for your review.
- Formal email (plural): All three theses have been submitted to the committee.
- Informal email: Just sent you my thesis draft. Let me know what you think.
Common Mistakes with the Plural of Thesis
Even advanced English learners sometimes make these errors. Here are the most frequent ones and how to fix them.
Mistake 1: Writing “Thesises”
Incorrect: She wrote two thesises for her degree.
Correct: She wrote two theses for her degree.
Why: Adding a regular -es to thesis creates a double plural. The word already has a special plural form.
Mistake 2: Writing “Thesis’s” or “Thesis’”
Incorrect: All the thesis’s were due on Friday.
Correct: All the theses were due on Friday.
Why: An apostrophe is used for possession, not for making a plural. Thesis’s means “belonging to one thesis,” not “multiple theses.”
Mistake 3: Using a Singular Verb with “Theses”
Incorrect: These theses is very long.
Correct: These theses are very long.
Why: Theses is plural, so it must take a plural verb.
Mistake 4: Confusing “Thesis” with “Hypothesis”
Incorrect: My thesis is that the drug works, and I tested it with two hypotheses.
Correct: My hypothesis is that the drug works, and I tested it with two experiments.
Why: A thesis is a long research paper. A hypothesis is a testable idea. They are different words, but both follow the same plural rule: hypothesis → hypotheses.
Better Alternatives and When to Use Them
Sometimes you might want to avoid using theses if you think your reader might find it confusing. Here are some alternatives depending on the context.
| Context | Alternative Phrase | When to Use It |
|---|---|---|
| Academic writing | dissertations | Use when referring to doctoral theses specifically. In some countries, thesis is for master’s and dissertation for PhD, but the plural rule is the same. |
| General writing | research papers | Use when you want a simpler word that everyone understands. It is less formal but perfectly clear. |
| Everyday speech | final projects | Use in casual conversation to avoid sounding too academic. For example: We all handed in our final projects last week. |
| Formal reports | submitted works | Use when listing multiple academic documents in a formal context. For example: The committee reviewed all submitted works. |
When you are writing formally, especially in academic or professional settings, it is best to use theses correctly. The alternatives are helpful for variety or clarity, but they do not replace the need to know the correct plural form.
Mini Practice: Test Yourself
Try these four questions. Write your answers down, then check below.
- Fill in the blank: She defended her ______ last semester. (singular)
- Fill in the blank: The professor read all the ______ carefully. (plural)
- Correct the error: Both thesises were well written.
- Choose the correct verb: This theses (is / are) about education reform.
Answers
- thesis — singular form is correct here.
- theses — plural form is needed because of all the.
- Both theses were well written. — Remove the extra -is.
- is — Wait, this is a trick. The sentence says This theses, which is incorrect. It should be This thesis is or These theses are. The correct verb depends on fixing the subject first. If you wrote is after changing theses to thesis, you are right. If you kept theses, you need are.
FAQ: Thesis Plural
1. Is “thesises” ever correct?
No. Thesises is not a word in standard English. The only correct plural is theses. Some learners create this form by mistake because they apply the regular plural rule to an irregular word.
2. How do you pronounce “theses”?
It is pronounced THEE-seez. The first syllable rhymes with see, and the second syllable rhymes with bees. The singular thesis is pronounced THEE-sis, with a short iss sound at the end.
3. Can “thesis” be used as a plural?
No. Thesis is always singular. Using it as a plural is a grammatical error. For example, I have three thesis to write is wrong. You must say I have three theses to write.
4. Do other words follow the same pattern?
Yes. Many Greek-derived words ending in -is change to -es in the plural. Common examples include crisis → crises, analysis → analyses, hypothesis → hypotheses, and parenthesis → parentheses. Learning this small group will help you avoid mistakes in academic and formal writing.
Final Tip for Remembering
Think of the word theses as having the same ending as these. Both are plural. If you can remember that these theses go together, you will never write thesises again. Practice using the word in your own writing, and check your work carefully before submitting any academic paper or email.
For more help with similar words, visit our Plural Spelling Rules section or explore Confusing Plurals for other tricky cases. If you have questions about this guide, please see our FAQ page or contact us.
