What Is the Plural of Thesis?
The plural of thesis is theses. This change follows the Greek-based plural rule where the singular ending -is becomes -es in the plural. You do not add an -s or -es in the usual English way. So, one thesis, two theses.
Quick Answer
Use theses when you are talking about more than one thesis. This applies in all contexts: academic writing, formal reports, casual conversation, and emails. There is no other correct plural form.
Why the Plural Is “Theses” and Not “Thesises”
Many English words that come from Greek or Latin keep their original plural patterns. Thesis is a Greek word, and in Greek, nouns ending in -is change to -es in the plural. Other common examples include crisis (crises), analysis (analyses), and hypothesis (hypotheses). If you try to write “thesises,” it will look like a mistake to most readers.
Formal vs. Informal Use
Theses is the standard plural in both formal and informal English. In a formal academic paper or a conference presentation, you will say “theses.” In a casual email to a classmate or a conversation with a friend, you still say “theses.” There is no shortened or slang version. The only difference is the context around it: formal writing often uses theses with precise references, while informal talk may use it more loosely.
Email Context
In an email to a professor or supervisor, you might write: “I have attached the first drafts of my two theses.” In a quick message to a colleague, you could say: “Can you check both theses before Friday?” The word stays the same.
Conversation Context
In conversation, you might hear: “She is working on three theses this semester.” Or: “We compared our theses last night.” The pronunciation changes slightly: thesis is pronounced /ˈθiː.sɪs/, while theses is pronounced /ˈθiː.siːz/. The vowel sound in the last syllable shifts from a short “is” to a long “eez.”
Comparison Table: Thesis vs. Theses
| Form | Number | Example |
|---|---|---|
| Thesis | Singular | My thesis is about climate change. |
| Theses | Plural | Both theses were submitted on time. |
| Thesis | Singular (possessive) | The thesis’s conclusion was strong. |
| Theses | Plural (possessive) | The theses’ authors met last week. |
Natural Examples
Here are examples that show how theses appears in real writing and speech:
- “The library holds over two hundred doctoral theses from the last decade.”
- “We need to revise our theses before the deadline.”
- “Her two theses cover very different topics: one on economics, one on history.”
- “The professor read all four theses over the weekend.”
- “In our department, students usually write their theses in the final year.”
Common Mistakes
Learners often make these errors with the plural of thesis:
- Mistake: “I have two thesis to finish.”
Correction: “I have two theses to finish.” - Mistake: “He wrote three thesises last year.”
Correction: “He wrote three theses last year.” - Mistake: “The thesis’ are due next week.” (using an apostrophe incorrectly)
Correction: “The theses are due next week.”
Remember: thesis never becomes thesises, and the plural does not use an apostrophe.
Better Alternatives and When to Use Them
Sometimes you might want to avoid repeating theses in a paragraph. Here are a few alternatives, but note that they are not exact synonyms:
- Dissertations – Often used for longer, more formal research projects, especially at the PhD level. In some countries, thesis and dissertation are interchangeable; in others, a thesis is for a master’s degree and a dissertation for a doctorate. Use dissertations when you want to emphasize the length or level of the work.
- Research papers – A broader term that can include shorter academic essays. Use this when the work is not a formal degree requirement.
- Studies – Useful when focusing on the research itself rather than the written document. For example: “Both studies reached similar conclusions.”
- Projects – A general term that works in informal contexts. For example: “We finished our final projects last month.”
When you are writing formally about academic requirements, theses is usually the best choice because it is precise.
Mini Practice: Theses or Thesis?
Fill in the blank with thesis or theses. Answers are below.
- She defended her __________ last Tuesday.
- The committee reviewed all three __________ carefully.
- How many __________ have you written so far?
- His __________ was rejected because of weak methodology.
Answers
- thesis
- theses
- theses
- thesis
Frequently Asked Questions
1. Is “thesises” ever correct?
No. “Thesises” is not a standard English word. The only correct plural is theses.
2. How do you pronounce “theses”?
It is pronounced /ˈθiː.siːz/ (THEE-seez). The last syllable rhymes with “seas” or “please.”
3. Can “thesis” be used as a plural in informal writing?
No. Even in informal writing, using thesis for more than one is incorrect. Always use theses for the plural.
4. What about the possessive form of “theses”?
The possessive of theses is theses’ (with an apostrophe after the s). For example: “The theses’ conclusions were contradictory.” The possessive of thesis is thesis’s (or sometimes thesis’ in older style guides).
Final Note
Remembering that thesis becomes theses will help you avoid a common error in academic writing. If you are unsure, think of other -is to -es words like crisis and analysis. Practice using theses in your own sentences, and it will soon feel natural.
For more help with similar plural forms, visit our Common Plural Forms section. You can also check our FAQ for quick answers to other questions.
