Common Mistakes with the Plural of Thesis
If you are writing an academic paper, a research summary, or even a formal email, you might need to use the word thesis more than once. The most common mistake is treating thesis as if it follows regular English plural rules. The correct plural is theses, not “thesises” or “thesis.” This guide explains why this happens, how to use the word correctly in different contexts, and how to avoid the errors that even advanced learners make.
Quick Answer: What Is the Plural of Thesis?
The plural of thesis is theses. It is an irregular plural that comes from Greek, not English. You change the -is ending to -es. So one thesis, two theses. Do not add an -es to the end of the word as you would with a regular noun like “box” (boxes).
Why Is the Plural Irregular?
Many English words borrowed from Greek and Latin keep their original plural forms. Thesis comes from Greek, where nouns ending in -is change to -es in the plural. Other examples include crisis (crises), analysis (analyses), and hypothesis (hypotheses). Once you learn this pattern, you can apply it to several academic words.
Comparison Table: Thesis vs. Theses
| Form | Meaning | Example Sentence |
|---|---|---|
| Thesis (singular) | One long research paper or central argument | Her thesis on climate policy was well received. |
| Theses (plural) | More than one research paper or argument | The library holds over two hundred doctoral theses. |
Formal vs. Informal Use
Thesis and theses are formal words. You will hear them most often in academic writing, university settings, research discussions, and formal correspondence. In everyday conversation, people might say “research papers” or “final projects” instead. However, if you are writing an email to a professor or submitting a manuscript, using the correct plural theses shows that you understand academic language.
In informal contexts, such as chatting with classmates, you might hear someone say “I have two theses to finish.” While this is grammatically correct, it can sound a little stiff. A more natural informal alternative would be “I have two big papers to finish.”
Natural Examples
Here are examples that show how thesis and theses appear in real writing and conversation.
Academic Writing
- The department requires all master’s theses to be submitted electronically.
- Each thesis must include a clear research question.
- She compared three theses from different universities.
Email Context
- Dear Professor Lee, I am attaching the revised version of my thesis.
- Could you please send me the guidelines for submitting doctoral theses?
Everyday Conversation
- “How many theses are you supervising this semester?” “Only two, but they are both very long.”
- “I finally finished my thesis!” “Congratulations. When do you defend it?”
Common Mistakes
Here are the three most frequent errors learners make with the plural of thesis.
Mistake 1: Adding -es to Make “Thesises”
This is the most common error. Because regular nouns form plurals by adding -es (e.g., bus → buses), learners often write “thesises.” This is incorrect. The correct form is theses.
Wrong: The library has many interesting thesises.
Right: The library has many interesting theses.
Mistake 2: Leaving the Word Unchanged
Some writers treat thesis as an uncountable noun or forget to change it at all. They write “two thesis” or “several thesis.” This is grammatically incorrect.
Wrong: She wrote three thesis during her PhD.
Right: She wrote three theses during her PhD.
Mistake 3: Confusing “Thesis” with “Hypothesis”
Both words follow the same plural pattern, but they mean different things. A thesis is a long paper or a central argument. A hypothesis is a testable prediction. Their plurals are theses and hypotheses. Do not mix them up.
Wrong: The scientist tested several theses.
Right: The scientist tested several hypotheses.
Better Alternatives and When to Use Them
Depending on your audience and purpose, you might choose a different word instead of theses. Here are some alternatives and the contexts where they work best.
| Alternative | Context | Example |
|---|---|---|
| Research papers | Informal or general academic discussion | I have to read three research papers this weekend. |
| Dissertations | Often used for doctoral-level work (UK and US) | Many dissertations are available online. |
| Final projects | Undergraduate or casual conversation | Our final projects are due next month. |
| Arguments | When referring to the central idea, not the paper itself | The two theses present conflicting arguments. |
Use theses when you are writing formally or when you need to be precise. Use research papers or final projects when speaking with classmates or in less formal settings.
Mini Practice: Test Yourself
Choose the correct word to complete each sentence. Answers are below.
- The professor has supervised more than twenty (thesis / theses) in her career.
- His (thesis / theses) on urban development won an award.
- We compared the main arguments of three different (thesis / theses).
- Are you planning to publish your (thesis / theses) after graduation?
Answers
- theses
- thesis
- theses
- thesis
Frequently Asked Questions
1. Is “thesis” singular or plural?
Thesis is singular. It refers to one paper or one central argument. The plural is theses.
2. Can I use “thesis” for both singular and plural?
No. Using “thesis” for more than one is grammatically incorrect. Always change it to theses for plural contexts.
3. How do I pronounce “theses”?
The singular thesis is pronounced THEE-sis. The plural theses is pronounced THEE-seez. The vowel sound in the second syllable changes from “is” to “eez.”
4. What other words follow the same pattern?
Many academic words from Greek follow this -is to -es pattern. Common examples include crisis (crises), analysis (analyses), hypothesis (hypotheses), parenthesis (parentheses), and diagnosis (diagnoses). Learning this group together can help you avoid mistakes.
Final Tip for Real Writing
When you are writing an email, a report, or a paper, take an extra second to check the plural of thesis. If you are talking about one, use thesis. If you are talking about two or more, use theses. This small change makes your writing look more polished and professional. If you are unsure, read your sentence aloud. If “thesises” sounds strange to you, that is a good sign you should use theses instead.
For more help with confusing plurals, visit our Confusing Plurals section. You can also check our Plural Spelling Rules for other patterns. If you have questions, see our FAQ or contact us.
