Common Mistakes with the Plural of Thesis
If you are writing an academic paper, a research summary, or even a formal email, you will likely need to use the word thesis more than once. The most common mistake is treating thesis like a regular English noun and adding an -es or -s to make it plural. The correct plural of thesis is theses (pronounced THEE-seez). This article explains why, gives you clear examples, and helps you avoid the errors that even advanced learners often make.
Quick Answer: What Is the Plural of Thesis?
The plural of thesis is theses. This follows the rule for many Greek-derived words ending in -is (like crisis → crises, hypothesis → hypotheses). Do not write thesises or thesi.
Why the Plural Is Not “Thesises”
English borrowed thesis directly from Latin, which took it from Greek. In Greek, nouns ending in -is change to -es in the plural. This is not a regular English pattern, so your instinct to add -es (as in box → boxes) is wrong here. Remembering this small group of words will instantly improve the accuracy of your academic writing.
Comparison Table: Thesis vs. Theses
| Form | Number | Example Sentence |
|---|---|---|
| thesis | singular | Her thesis on climate change was well received. |
| theses | plural | The department reviewed three theses this semester. |
Natural Examples of “Thesis” and “Theses”
Seeing the word used in real contexts helps you internalize the correct form. Below are examples for different situations.
In Academic Writing (Formal)
- The library now holds over two hundred doctoral theses from the last decade.
- Each thesis must include a clear methodology section.
- The committee read both theses before the defense.
In Email or Conversation (Semi-formal to Informal)
- I am still working on my thesis, so I cannot join the trip.
- Have you seen the theses from last year’s graduates? They are online now.
- My friend finished her thesis last week, and she is finally relaxing.
Nuance: When to Use “Thesis” vs. “Theses”
Use thesis when talking about one argument, paper, or central idea. Use theses when referring to multiple papers or multiple central arguments. In formal contexts, such as a research symposium, you might hear: “The theses presented today cover diverse topics.” In a casual chat, you might say: “My thesis is taking forever to write.” The tone does not change the grammar—only the number does.
Common Mistakes with the Plural of Thesis
Below are the three most frequent errors learners make, along with corrections.
Mistake 1: Adding -es (Thesises)
Incorrect: The professor read all three thesises carefully.
Correct: The professor read all three theses carefully.
Mistake 2: Keeping the Singular Form for Multiple
Incorrect: There are two thesis on the table.
Correct: There are two theses on the table.
Mistake 3: Using “Thesi” as a Plural
Incorrect: The library has many interesting thesi.
Correct: The library has many interesting theses.
Better Alternatives and When to Use Them
Sometimes you may want to avoid the word theses altogether, especially if you are unsure of the spelling or if the context calls for a different term. Here are some alternatives.
| Alternative | When to Use It | Example |
|---|---|---|
| dissertations | When referring to long, formal doctoral research papers | The university published five dissertations this year. |
| research papers | In general academic or casual contexts | She wrote several research papers during her master’s. |
| arguments | When focusing on the central claim, not the document | The two arguments in the article contradict each other. |
| proposals | When the work is still in the planning stage | All proposals must be submitted by Friday. |
Use theses when you are specifically talking about the formal academic documents submitted for a degree. Use arguments or claims when you mean the central ideas within any piece of writing.
Mini Practice: Test Yourself
Choose the correct word to complete each sentence. Answers are below.
- The university requires all master’s __________ to be submitted online. (thesis / theses)
- Her __________ on medieval poetry won an award. (thesis / theses)
- We compared three different __________ for our literature review. (thesis / theses)
- One __________ argued that the data was inconclusive. (thesis / theses)
Answers
- theses (plural, referring to multiple submissions)
- thesis (singular, referring to one person’s work)
- theses (plural, comparing multiple documents)
- thesis (singular, one argument)
Frequently Asked Questions
1. Is “thesis” singular or plural?
Thesis is singular. Its plural is theses. Never use thesis to refer to more than one.
2. How do you pronounce “theses”?
Theses is pronounced THEE-seez (two syllables, with a long e sound at the end). This is different from thesis (THEE-sis), which ends with a short is sound.
3. Can I use “thesis” for both singular and plural in informal writing?
No. Even in informal emails or messages, using thesis for multiple items sounds incorrect and may confuse your reader. Stick with theses for plural.
4. What other words follow the same pattern as thesis/theses?
Many academic and scientific words follow this pattern. Common examples include crisis → crises, hypothesis → hypotheses, analysis → analyses, parenthesis → parentheses, and diagnosis → diagnoses. Learning this small group will help you avoid many plural mistakes.
Final Tip for Real Writing
When you are proofreading your own work, search for the word thesis. If you are talking about more than one, change it to theses. This one change can make your writing look much more polished. For more help with confusing plurals, visit our Confusing Plurals section. You can also review Plural Spelling Rules for other tricky words. If you have questions about your own writing, feel free to contact us.
