Whats the Plural of Thesis: Understanding Singular and Plural Forms
Quick Answer
The plural of thesis is theses. The word “thesises” is not correct in English.
One: She wrote a long thesis for her degree.
More than one: The students handed in their theses last week.
Thesis Becomes Theses — See the Pattern
Look at these words. Can you see what happens?
- one thesis → two theses
- one crisis → two crises
- one analysis → two analyses
- one oasis → two oases
Did you see the pattern? The -is at the end changes to -es. This rule works for many English words that come from Greek.
| One (singular) | More than one (plural) |
|---|---|
| thesis | theses |
| crisis | crises |
| analysis | analyses |
| basis | bases |
| hypothesis | hypotheses |
Why It Is Theses and Not Thesises
In English, many words that end in -sis or -is come from the Greek language. These words do not add -es to the end like normal plurals. Instead, the -is changes to -es:
- thesis → theses (not “thesises”)
- crisis → crises (not “crisises”)
- parenthesis → parentheses (not “parenthesises”)
A thesis is a long piece of writing that a student does for a university degree. It can also mean a main idea or argument. You say it like “THEE-sis” for one, and “THEE-seez” for more than one.
Easy way to remember: Think of crisis → crises. Most people know that one. The word thesis works the same way: thesis → theses.
The Rule in One Line: Thesis ends in -is, so the plural is theses (-is → -es).
How to Use Thesis and Theses in Everyday English
- She finished her thesis after two years of work. (one thesis)
- The library keeps all the old theses from past students. (more than one)
- His thesis was about how children learn languages. (one thesis)
- The professor read five theses before the meeting. (more than one)
- My thesis is that reading every day helps you learn faster. (one main idea)
You are doing great. Now let us look at the mistake most learners make with this word.
The Most Common Mistake With Thesis
Many learners write “thesises” because most English plurals just add -s or -es to the end. That is a smart guess — but it does not work here. Even native speakers sometimes pause and think about this one. So if you get confused, you are not alone.
✗ The students submitted their thesises last Friday.
✓ The students submitted their theses last Friday.
✗ I read three thesises about the same topic.
✓ I read three theses about the same topic.
✗ How many thesises did you write?
✓ How many theses did you write?
How to remember: Think of it like this — the word ends in -sis. If you see -sis at the end, change it to -ses. Thesis → theses. Simple.
Other words like this: crisis → crises, analysis → analyses, basis → bases, hypothesis → hypotheses, oasis → oases.
Test Yourself: Thesis or Theses?
Choose the correct answer for each sentence. Click Check to see if you are right.
1. She spent two years writing her _______.
2. The professor reviewed all the _______ before the meeting.
3. The library has more than 500 _______ from past students.
4. His _______ is that children learn best through play.
5. Both of their _______ were about the same topic.
Keep Going — You Are Building Something
You just learned that thesis becomes theses. That is one more plural you will never get wrong again.
But here is something interesting. You now know that words ending in -sis change to -ses. But what about the word hypothesis? Does it follow the same rule, or is there a surprise? And when would you even use this word in real life?
Next lesson: What Is the Plural of Hypothesis?
Sources
Definition of thesis — Cambridge Dictionary
My name is Khamis Maiouf. I am the creator of the English Teacher Site, dedicated to providing valuable resources and insights for students around the world. With a passion for education and a commitment to helping students enhance their skills, I aim to make English teaching more effective and enjoyable for both educators and students.
