Plural of “parenthesis”

The plural of parenthesis is parentheses.

Why

"Parenthesis" comes from Greek "parenthesis," literally "a placing in beside" (from "para-," beside, and the same "thesis" root meaning "a placing"), and it follows the standard Greek -is/-es plural pattern: the plural is pronounced with a full extra syllable, "puh-REN-thuh-seez."

The word technically refers to the inserted explanatory or qualifying phrase itself ("the aside, set off in parenthesis"), while the curved punctuation marks that typically enclose it, ( and ), are properly called "parentheses" as well — meaning the plural noun does double duty for both the grammatical device and its typical punctuation marks, a genuine overlap in meaning rather than a simple coincidence of spelling. Legal and technical writing occasionally uses "in parenthesis" as a fixed adverbial phrase describing information added incidentally rather than as a literal reference to punctuation marks at all, an abstract, idiomatic extension of the word's core meaning that nonetheless still follows the identical grammar discussed throughout this entry.

Usage note

In everyday writing and speech, most people use "parentheses" loosely to mean the punctuation marks themselves ("put that in parentheses"), while "parenthesis" in the singular is comparatively rare outside grammar instruction, since a single curved mark alone isn't typically what a writer refers to — it's the paired set, or the enclosed aside, that gets named.

"Parenthetical," the adjective derived from "parenthesis," is fully regular and has broadened in meaning well beyond punctuation to describe any incidental or secondary remark in speech or writing ("a parenthetical comment"), a figurative extension distinct from the strict grammatical sense the plural noun itself still carries in formal usage. Style guides for legal writing specifically warn against overusing parentheses to smuggle in qualifying information a sentence should really state directly, a stylistic concern distinct from, but frequently raised alongside, the grammar of the word's plural form. Some writers use a single parenthesis mark alone in informal digital writing, such as a lone closing mark to suggest a wry or ironic tone, a distinctly modern, casual usage far removed from the word's traditional grammatical sense.

Common real mistakes

Calling a single curved mark, ( or ), "a parenthesis" is grammatically defensible but sounds unusual to most readers, who expect "parenthesis" to describe the whole enclosed unit rather than one lone mark; a more common real error is simply misspelling or mispronouncing the plural the same as the singular, the same slip seen across this entire Greek word family. A further error worth noting: some writers mistakenly refer to square brackets, [ and ], as "parentheses" as well, when careful usage reserves "parentheses" specifically for the curved marks and uses distinct terms ("brackets," "braces") for the other paired punctuation marks used in formal and technical writing.

The wider pattern

"Parenthesis/parentheses" shares its Greek -is/-es pattern with "thesis/theses" and "hypothesis/hypotheses," all three built on the same underlying Greek root for "placing," though "parenthesis" is unusual for how thoroughly its plural has come to refer to punctuation marks in everyday usage rather than strictly to the grammatical device.

Examples

• She added a brief aside in parenthesis to clarify the date.
• The sentence contained two separate parentheses, one nested inside the other.
• Editors often flag overused parentheses as a sign of cluttered prose.
• He closed the parenthesis before finishing the rest of the sentence.
• The style guide recommends minimizing nested parentheses wherever possible.
• Both parentheses in the paragraph interrupted the sentence's flow.
• A well-placed parenthesis can add helpful context without derailing a sentence.
• The manuscript was full of unnecessary parentheses that slowed the reading.
• She removed one of the two parentheses to simplify the sentence.
• The footnote appeared entirely within a single parenthesis.
• The editor removed several unnecessary parentheses to tighten the paragraph.
• Legal contracts frequently use parentheses to define specialized terms on first use.
• The professor advised students to avoid burying key arguments inside parentheses.