What’s the Plural of Parenthesis: Understanding English Grammar

Parenthesis or Parentheses — See the Pattern

Look at these sentences. Can you see how the word changes?

• There is one parenthesis missing from this sentence. (just one bracket)
• Please put the dates in parentheses. (a pair of brackets)
• The opening parenthesis goes before the extra information. (one bracket)
• I counted five sets of parentheses in that paragraph. (more than one)

Did you see the pattern? When there is one, you say parenthesis. When there are two or more, you say parentheses. The ending changes from -is to -es.

When to Use Parenthesis and When to Use Parentheses

A parenthesis is one of these curved brackets: ( or ). It is also the extra information you add inside the brackets.

Use parenthesis (singular) when you mean:

• One bracket — “Add an opening parenthesis before the number.”
• One piece of extra information — “The writer used a parenthesis to explain the word.”

Use parentheses (plural) when you mean:

• A pair of brackets — “Put the year in parentheses.”
• More than one set — “The report has too many parentheses.”

This word comes from Greek. In Greek, words that end in -sis change to -ses when they become plural. English borrowed this rule, so we do the same thing.

Easy way to remember: Brackets always come in pairs — and pairs need the plural: parentheses. If you are talking about the pair ( ), you almost always need parentheses.

How to Use Parentheses in Everyday English

• The teacher wrote the answer in parentheses at the end. (she used a pair of brackets)
• I forgot to close the parenthesis at the end of the sentence. (one missing bracket)
• His email was full of parentheses — it was hard to read. (many sets of brackets)
• Put your phone number in parentheses after your name. (a pair of brackets)
• The first parenthesis should go right before the date. (one bracket)

You are doing great. Now let us look at the mistakes many learners make.

The Most Common Mistake With Parenthesis

Even advanced learners get confused by this one — so if you mix up parenthesis and parentheses, you are not alone. Most English plurals just add -s, so this Greek-style change feels strange at first.

How to remember: Think of the word thesis — its plural is theses. The same pattern works for parenthesis → parentheses. If the singular ends in -sis, the plural ends in -ses.

Other words like this: thesis → theses, crisis → crises, analysis → analyses, basis → bases, diagnosis → diagnoses, oasis → oases.

Test Yourself: Parenthesis or Parentheses?

Choose the correct answer for each sentence. Click Check to see if you are right.

Keep Going — You Are Building Something

You just learned how to make parenthesis plural. That is one more Greek-style plural you will never get wrong again.

But here is a question — the word analysis follows the exact same pattern. Do you know what its plural is? It is not analysises. Most people get this one wrong too.

Next lesson: What Is the Plural of Analysis?

Sources

Origin of parenthesis — Etymonline

Grammarist — Parentheses

Merriam-Webster — Definition of parenthesis