Thesis Plural Form: Rule, Examples, and Mistakes
The plural form of thesis is theses (pronounced THEE-seez). This follows the Greek-based rule for nouns ending in -is, where the -is changes to -es in the plural. Unlike regular English nouns that simply add -s or -es, thesis is an irregular plural that requires a spelling and pronunciation shift. Using thesises or thesi is incorrect in standard academic English.
Quick Answer
If you need a fast reference:
- Singular: thesis
- Plural: theses
- Pronunciation: singular = THEE-sis, plural = THEE-seez
- Rule: Change -is to -es (Greek plural pattern)
- Common error: Do not write thesises or thesis’ as a plural
Why the Plural Is “Theses”
English borrowed thesis directly from Greek, where the singular ends in -is and the plural ends in -es. This pattern appears in several academic words, such as crisis (crises), hypothesis (hypotheses), analysis (analyses), and parenthesis (parentheses). Because these words entered English through scholarly Latin and Greek routes, they kept their original plural forms rather than adopting the regular English -s or -es endings.
When you write about multiple theses, you are referring to more than one long research paper submitted for a degree, or more than one central argument in a discussion. The spelling change signals that shift clearly in academic writing.
Formal vs. Informal Use
Formal academic writing: Always use theses as the plural. This is the standard in university papers, dissertations, journal articles, and conference presentations. For example: The department reviewed three theses this semester.
Informal conversation or email: Even in casual academic chat, stick with theses. Native speakers rarely use an incorrect form like thesises in speech, but they might say thesis papers as a workaround. For instance: I have to read five thesis papers by Friday. This is acceptable in informal contexts but less precise in formal writing.
Email context: In professional emails to professors or colleagues, use theses to maintain credibility. Example: Could you send me the theses from last year’s cohort?
Comparison Table: Thesis vs. Theses
| Feature | Singular: Thesis | Plural: Theses |
|---|---|---|
| Meaning | One long research paper or one central argument | Multiple research papers or multiple arguments |
| Pronunciation | THEE-sis | THEE-seez |
| Spelling pattern | Ends in -is | Ends in -es |
| Grammar rule | Irregular singular | Irregular plural (Greek origin) |
| Example sentence | Her thesis on climate change won an award. | Both theses were submitted on time. |
| Common mistake | Using thesis for multiple papers | Writing thesises or thesis’ |
Natural Examples
Here are real-world sentences showing how thesis and theses appear in different contexts:
- Academic writing: The professor assigned three theses for the seminar discussion.
- Graduate program: Each student must defend their thesis before the committee.
- Library catalog: All theses from the Department of History are available online.
- Conference talk: Her thesis argues that economic growth does not always reduce inequality.
- Email to advisor: I am attaching the revised version of my thesis for your feedback.
- Group project: We compared the theses of three different authors on the same topic.
Common Mistakes
Learners and even native speakers sometimes make errors with thesis. Here are the most frequent ones:
Mistake 1: Using “thesises”
Incorrect: I have to read five thesises this month.
Correct: I have to read five theses this month.
Why: English does not add -es to an already irregular plural. The Greek pattern already changes the ending.
Mistake 2: Using “thesis” as a plural
Incorrect: The library holds many thesis on different subjects.
Correct: The library holds many theses on different subjects.
Why: The singular form cannot refer to multiple items. The spelling must change to show plurality.
Mistake 3: Adding an apostrophe for plural
Incorrect: All the thesis’ were graded last week.
Correct: All theses were graded last week.
Why: Apostrophes indicate possession, not plurality. Thesis’ would mean “belonging to one thesis.”
Mistake 4: Confusing pronunciation
Incorrect: Saying THEE-sis for both singular and plural.
Correct: Singular = THEE-sis, Plural = THEE-seez.
Why: The vowel sound changes from a short i to a long ee sound in the plural. This helps listeners distinguish between one and many.
Better Alternatives and When to Use Them
Sometimes you may want to avoid the irregular plural altogether, especially in informal contexts. Here are alternatives:
- Thesis papers: Use this in casual conversation or email. Example: I need to review three thesis papers tonight. This is clearer for non-academic listeners.
- Dissertations: In some countries, thesis refers to a master’s level paper, while dissertation refers to a doctoral paper. Use dissertations if you mean PhD-level work. Example: The library has a collection of doctoral dissertations.
- Research papers: A broader term that includes theses but also shorter academic articles. Example: All research papers must follow the same format.
- Arguments: When thesis means a central claim, you can say main arguments or central claims. Example: The authors present three main arguments in their book.
When to use it: Stick with theses in formal academic writing, grant proposals, and university communications. Use thesis papers or dissertations in everyday speech or emails to avoid confusion. For the “central argument” meaning, theses is still correct but less common; arguments often works better.
Mini Practice: Check Your Understanding
Test yourself with these four questions. Answers are below.
- What is the correct plural of thesis?
a) thesises
b) theses
c) thesis’
d) thesi - Which sentence is correct?
a) The professor assigned three thesis for the course.
b) The professor assigned three theses for the course.
c) The professor assigned three thesises for the course.
d) The professor assigned three thesis’ for the course. - How do you pronounce the plural of thesis?
a) THEE-sis
b) THEE-seez
c) THES-is
d) THES-eez - Which word is a good informal alternative to theses?
a) thesis papers
b) thesises
c) thesis’
d) thesi
Answers
- b) theses – This is the only correct plural form.
- b) The professor assigned three theses for the course. – The plural must be theses.
- b) THEE-seez – The plural ends with a long eez sound.
- a) thesis papers – This is a clear, informal alternative.
Frequently Asked Questions
1. Is “thesises” ever correct?
No. Thesises is not a standard English word. It appears in some learner errors but is never accepted in academic or professional writing. Always use theses.
2. Can I use “thesis” as both singular and plural?
No. Using thesis for multiple papers is grammatically incorrect. You must change the spelling to theses for the plural. This is similar to crisis (crises) and analysis (analyses).
3. What is the possessive form of “thesis”?
The singular possessive is thesis’s (e.g., the thesis’s conclusion) or thesis’ (e.g., the thesis’ argument). Both are acceptable, though thesis’s is more common in modern style guides. The plural possessive is theses’ (e.g., the theses’ findings).
4. Does “theses” only refer to academic papers?
No. Thesis can also mean a central argument or proposition in a debate or essay. In that sense, theses refers to multiple arguments. For example: The philosopher presented several theses in his lecture. This usage is common in philosophy, logic, and rhetoric.
Final Note on Spelling Rules
If you are learning English plural forms, remember that words ending in -is that come from Greek usually change to -es. This is one of the plural spelling rules that applies to academic vocabulary. Other examples include hypothesis (hypotheses), crisis (crises), and parenthesis (parentheses). Mastering this pattern will help you avoid common errors in your writing.
For more help with similar words, explore our guides on common plural forms and confusing plurals. If you have questions about specific usage, visit our FAQ page or contact us for clarification.
