Beyond the brackets: why "a parentheses" is a mistake

What kind of error is it?

Singular vs. plural usage error

Why do people make this mistake?

People often confuse the singular and plural forms of words, especially when the plural form doesn't just add an "s". "Parentheses" is one such word, where the singular is "parenthesis", and the plural is "parentheses". Since both forms sound similar, mixing them up is a common mistake.

What is correct?

"Parenthesis" is the correct singular form of the noun, referring to one of the curved marks (like this: "(" or ")"). "Parentheses" is the correct plural form, referring to the pair.