The singular form is parenthesis, but the plural parentheses is the word you’re more likely to see. Both words have a wide range of related meanings, and what some people identify as a parenthesis, others call parentheses.

So let’s keep it simple. For our purposes, a parenthesis is one of a pair of curved marks that look like this: ( ), and parentheses are both marks.

A symbol, number, letter, word, phrase, or clause that is in parentheses explains, supplements, or comments on something in the sentence. Material in parentheses can be removed from a sentence without changing that sentence’s overall meaning or grammatical integrity.

Note the use of is in this sentence: My friend (and her brother) is coming today. The subject is My friend. Despite appearances, parentheses are never part of the subject. Remove them and we’d have two subjects, My friend and her brother, which would require the verb are coming. The use of parentheses is a clue that the writer was more concerned about the friend than about the brother.

Parentheses, long dashes, and commas are the three punctuation marks that indicate an interruption in the flow of a sentence. (Some might add semicolons, which can turn two simple sentences into a single, more complex sentence: Their eyes met; she smiled.)

Commas, the least intrusive of the three, signal the presence of relevant but nonessential data. Long dashes either expand upon the main point or take a slight detour from it. Parentheses by their very appearance let the reader know that the information fenced off by those vertical curves is a departure from the rest of the sentence. To illustrate:

• Blaine, who was born in 1797 and died in 1860, did not live to see the Civil War.

• Blaine—he was born in 1797 and died in 1860—did not live to see the Civil War.

• Blaine (1797-1860) did not live to see the Civil War.

Sometimes the choice is clear. For instance, you’d never see this sentence: Blaine—1797-1860—did not live to see the Civil War. But it is also true that a writer’s use of one of these marks instead of another is often a matter of personal taste.

Parentheses can be used to form a separate sentence, as here: I hoped my friend was coming. (He canceled at the last minute.) But the writer could also have done this: I hoped my friend was coming (he canceled at the last minute). Note the placement of the period; if parentheses end a sentence, the period goes after the closing parenthesis.

Commas virtually always follow parentheses rather than precede them. This sentence is incorrect: When he got home, (it was already dark outside) he fixed dinner. Make it When he got home (it was already dark outside), he fixed dinner.

Writers have a lot of leeway with parentheses, as long as they heed a few simple guidelines. Used shrewdly (and sparingly!), parentheses add color, nuance, and spice to your writing.