Parentheses or Parenthesis: Which One Should You Use? in 2026

The Short answer is : Parenthesis is singular, and parentheses is plural. 

I remember writing an academic paragraph and typing, “Add this inside the parenthesis.” My teacher corrected it to “parentheses,” and I felt confused. Why did the whole word change instead of just adding an -s? 

Many people search “parentheses or parenthesis” because the spelling shift looks unusual. Unlike regular plurals, this word comes from Greek and follows a different pattern. 

In this guide, I’ll clearly explain the meaning, grammar rule, correct usage, and how to avoid common mistakes in writing.

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Parentheses or Parenthesis : Quick Answer

Parenthesis = one curved punctuation mark used to add extra information ( )
Parentheses = two curved punctuation marks used together

Meaning: These marks are used to include additional or clarifying information in a sentence.

Examples:

• Add the note inside the parenthesis. (referring to one mark)
• Put the extra details in parentheses. (referring to the pair)

In most cases, you will use parentheses because the marks normally come in pairs.

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The Origin of Parentheses and Parenthesis

The word comes from Greek parentithenai, meaning “to place beside.”

Many Greek-origin words change in special ways when plural:

• analysis → analyses
• crisis → crises
• thesis → theses
• parenthesis → parentheses

Instead of adding -s, the ending changes from -is to -es.

So:

• parenthesis (singular)
• parentheses (plural)

This is a grammar pattern, not a spelling mistake.

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What Do Parentheses Mean in Writing?

Parentheses are punctuation marks used to:

• Add extra information
• Include explanations
• Give examples
• Insert dates or references

Example:

• Ali was born in Lahore (Pakistan).
• She finally answered the question (after five minutes of silence).

The information inside parentheses is usually not essential to the main sentence.

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British English vs American English Usage

There is no spelling difference between British and American English for these words.

Both use:

• parenthesis (singular)
• parentheses (plural)

Here is a quick comparison:

Form	American English	British English
Singular	parenthesis	parenthesis
Plural	parentheses	parentheses
Meaning	Punctuation mark	Punctuation marks

Unlike words such as defense/defence, there is no regional spelling change here.

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Which Word Should You Use?

In normal writing, you will almost always use parentheses, because punctuation marks come in pairs.

Use parenthesis only when:

• Talking about one single curved mark
• Referring to the grammatical concept in technical discussion

Examples:

• The sentence contains a misplaced parenthesis.
• Remove the extra parenthesis at the end.

But in general writing:

• Add the explanation in parentheses.
• The details appear in parentheses.

If you are unsure, ask yourself:
Are you talking about one mark or a pair?

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Common Mistakes with Parentheses or Parenthesis

1. Using Singular When You Mean Plural

❌ Put the answer in parenthesis.
✅ Put the answer in parentheses.

2. Adding “s” Incorrectly

❌ parenthesiss
❌ parenthesises
✅ parentheses

3. Confusing With Brackets

Parentheses: ( )
Brackets: [ ]

They are not the same punctuation marks.

4. Forgetting Verb Agreement

❌ The parentheses is misplaced.
✅ The parentheses are misplaced.

Remember:

• Parenthesis → is
• Parentheses → are

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Parenthesis or Parentheses in Everyday Examples

In Academic Writing

• The formula is explained in parentheses.
• A closing parenthesis is missing.

Emails

• Please include your phone number in parentheses.
• There is an extra parenthesis in this sentence.

In Books

• The author added a comment in parentheses.
• The editor corrected the misplaced parenthesis.

On Social Media

• Just kidding (obviously).
• The joke was explained in parentheses.

Most of the time, you will see parentheses used.

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Parentheses in Grammar and Mathematics

In mathematics, parentheses group numbers:

• (5 + 3) × 2

Grammar, parentheses add extra information:

• The meeting (which lasted two hours) was productive.

In both cases, the plural parentheses is commonly used because they appear as a pair.

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Comparison Table: Parenthesis vs Parentheses

Feature	Parenthesis	Parentheses
Number	Singular	Plural
Meaning	One curved mark	A pair of curved marks
Verb Used	is	are
Common Usage	Rare	Very common
Region Difference	None	None

The main difference is singular versus plural.

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Frequently Asked Questions (FAQs)

1. What is the difference between parenthesis and parentheses?

Parenthesis is singular. Parentheses is plural.

2. Which one is more commonly used?

Parentheses is more commonly used because punctuation marks come in pairs.

3. Is parentheses singular?

No. Parentheses is plural.

4. Why does parenthesis change to parentheses?

It follows a Greek-origin plural rule where -is changes to -es.

5. Can I say “in parenthesis”?

Grammatically, yes, if referring to one mark. But normally you say “in parentheses.”

6. Is there a spelling difference between US and UK?

No. Both use the same spelling.

7. What verb should I use with parentheses?

Use “are” because it is plural.

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Conclusion

The difference between parenthesis and parentheses is about number. Parenthesis refers to one curved punctuation mark. Parentheses refers to the pair used in writing. Because these marks normally appear together, the plural form is much more common. The unusual spelling change happens because the word comes from Greek and follows a special plural pattern where -is becomes -es.

There is no regional spelling difference between American and British English. The only thing you need to remember is singular versus plural and matching the correct verb. If you are writing general sentences, you will almost always use parentheses. Understanding this pattern will also help you with other similar words like analysis/analyses and crisis/crises.

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