The plural form of parenthesis is parentheses. While the singular noun ends in -is, the plural form replaces this ending with -es. This linguistic shift is not a random quirk of English spelling but a reflection of the word's ancient Greek origins.

In everyday writing, you are much more likely to encounter the plural form, parentheses, because these punctuation marks almost exclusively appear in pairs. However, understanding the distinction between the singular and plural is essential for grammatical precision, especially in technical, mathematical, and linguistic contexts.

The Etymology Behind the Pluralization

To understand why parenthesis becomes parentheses, one must look at its history. The word is a learned borrowing from Late Latin, which in turn took it from the Ancient Greek parénthesis, meaning "a putting in beside" or "insertion."

In Greek-derived English nouns, the suffix -is often indicates a singular noun, and the standard method for pluralizing these words is to change the -is to -es. This is a distinct category of irregular plurals in English. If you memorize this pattern, you will find it applies to a wide array of academic and scientific terms:

  • Analysis becomes analyses
  • Basis becomes bases
  • Crisis becomes crises
  • Diagnosis becomes diagnoses
  • Thesis becomes theses
  • Hypothesis becomes hypotheses

In the case of parenthesis, the transformation follows this exact historical trajectory. When you are referring to one of the two curved marks, you are referring to a parenthesis. When referring to the set, or the punctuation category in general, you use parentheses.

Significant Differences in Pronunciation

A common mistake in spoken English is failing to distinguish between the singular and plural pronunciations of this word. Because the spelling change is subtle—only one letter—speakers often overlook the phonetic shift.

  1. Parenthesis (Singular): The final syllable is pronounced with a short "i" sound, similar to "hiss." The phonetic transcription is /pəˈrɛnθəsɪs/.
  2. Parentheses (Plural): The final syllable shifts to a long "e" sound, rhyming with "trees" or "sees." The phonetic transcription is /pəˈrɛnθəsiːz/.

In professional settings, such as linguistics lectures or editorial meetings, using the correct plural pronunciation demonstrates a high level of literacy and attention to detail.

Parenthesis as a Punctuation Mark versus a Grammatical Device

It is important to distinguish between the physical marks on a page and the grammatical function they perform.

The Punctuation Mark

In American English, the term parentheses refers to the curved marks ( ). In British English, these are frequently called round brackets to distinguish them from square brackets [ ] or curly brackets { }.

When you place a word or phrase inside these marks, you are creating a parenthetical statement. If you are discussing the mechanics of the symbols themselves—for example, "The left parenthesis is missing from this equation"—the singular form is correct.

The Grammatical Device

Beyond the marks themselves, a parenthesis (singular) refers to an explanatory or qualifying word, clause, or sentence inserted into a passage. Crucially, a parenthesis is grammatically independent of the rest of the sentence. If you were to remove it, the surrounding sentence would still be complete and correct.

For example: "The discovery (which was made quite by accident) changed the course of the study." In this sentence, the text within the marks is the parenthesis. You could also set off this same parenthesis using commas or em dashes:

  • "The discovery, which was made quite by accident, changed the course of the study."
  • "The discovery—which was made quite by accident—changed the course of the study."

Professional Applications of Parentheses

The usage of parentheses varies significantly across different disciplines. Understanding these nuances is critical for anyone working in specialized fields.

Mathematics and Logic

In mathematics, parentheses are used to group terms and define the order of operations. According to the PEMDAS/BODMAS rules, operations inside parentheses are performed first.

  • In the expression 3 * (4 + 5), the parentheses indicate that 4 and 5 must be added before multiplying by 3.
  • In formal logic, parentheses clarify the scope of quantifiers and connectives, preventing ambiguity in complex propositions.

Computer Programming and Syntax

For software engineers, parentheses are foundational elements of syntax. Most programming languages (C++, Java, Python, JavaScript) use parentheses to:

  • Enclose parameters in function definitions: def calculate_area(radius):
  • Invoke functions: print("Hello World")
  • Control the flow of conditional statements: if (x > 10) { ... }

In the Lisp family of languages, parentheses are so ubiquitous that the language is often joked about as "Lost In Stupid Parentheses," though they are the very mechanism that allows the language to treat code as data.

Legal and Academic Writing

In legal documents, parentheses are often used to provide citations or to define shorthand terms that will be used throughout a contract. For instance: "The International Business Corporation (hereinafter referred to as 'the Company') agrees to..."

In academic writing, particularly following APA or MLA styles, parentheses are the standard vessel for in-text citations. Using them correctly—and knowing whether to refer to them as a singular parenthesis or plural parentheses in your methodology—is a hallmark of scholarly competence.

Rules for Punctuating With Parentheses

One of the most frequent questions writers ask is how to handle other punctuation marks, like periods and commas, when they encounter a parenthesis at the end of a sentence.

The Period Rule

If the parentheses enclose a fragment at the end of a sentence, the period goes outside the closing parenthesis.

  • Correct: The project was finished on time (despite the setbacks).
  • Incorrect: The project was finished on time (despite the setbacks.)

However, if the parentheses enclose an entire independent sentence, the period goes inside.

  • Correct: The team was exhausted. (They had been working for eighteen hours straight.)

The Comma Rule

A comma should never precede a parenthesis. If a comma is required for the sentence structure, it should always follow the closing parenthesis.

  • Correct: After the meeting ended (at 5 PM), we went to dinner.
  • Incorrect: After the meeting ended, (at 5 PM) we went to dinner.

Stylistic Considerations: When to Use Parentheses

While parentheses are useful for adding "bonus" information, overusing them can make text feel fragmented and difficult to read. As an editor, I often advise writers to simulate the "whisper test." A parenthesis is like a stage whisper—it’s an aside meant for the audience that doesn't disrupt the main action.

If the information is vital to the reader's understanding, it shouldn't be in parentheses. If it’s a strong interruption, consider em dashes. If it’s a smooth, integrated detail, consider commas. Use parentheses when the information is truly supplemental, such as dates, citations, or brief clarifications.

Summary of Parenthesis vs. Parentheses

Term Form Typical Usage Pronunciation
Parenthesis Singular Refers to one mark ( or ) or one specific aside. Ends in "hiss"
Parentheses Plural Refers to the pair ( ) or the general punctuation category. Ends in "heeze"

Frequently Asked Questions

What is the plural of parenthesis?

The plural form is parentheses. It follows the Greek-to-English pluralization rule where the -is ending changes to -es.

Can I say "parenthesises"?

No, "parenthesises" is grammatically incorrect. While many English words add "-es" or "-s" to form a plural, words of Greek origin ending in "-is" are an exception.

Is it "in parenthesis" or "in parentheses"?

Both can be correct depending on the context, but "in parentheses" is much more common when referring to text placed inside the marks. "In parenthesis" is often used idiomatically in British English to mean "as an aside" or "incidentally."

Are parentheses and brackets the same thing?

In American English, "parentheses" specifically refers to the curved ( ) marks, while "brackets" usually refers to square [ ] marks. In British English, "brackets" is a general term, and ( ) are called "round brackets."

How do I use parentheses in a sentence?

Use them to include information that is not essential to the main point. Ensure that if you removed the parentheses and the text inside them, the sentence would still make complete sense on its own.

Should I capitalize the first word inside parentheses?

Only if the content inside the parentheses is a complete, independent sentence that stands alone between other sentences. If the parenthetical information is tucked inside another sentence, do not capitalize the first word unless it is a proper noun.

Where does the question mark go with parentheses?

If the parenthetical text itself is a question, the question mark stays inside. If the entire sentence is a question and the parentheses just happen to be at the end, the question mark goes outside.

  • Example 1: Do you like the new office (the one on 5th Street)?
  • Example 2: He finally arrived (but was he late?).

Conclusion

Mastering the plural form of parenthesis is a small but vital step in achieving grammatical excellence. By recognizing the Greek roots that dictate the shift to parentheses, you not only improve your spelling but also gain a deeper understanding of the internal logic of the English language. Whether you are coding a complex algorithm, solving an algebraic equation, or writing a persuasive essay, using parentheses with precision ensures that your asides and supplementary details enhance—rather than obscure—your message.