in parenthesis (in parentheses)

Most writers mean the pair of curved marks when they write "in parenthesis." The correct plural is "parentheses." Below are clear rules, spacing and hyphenation tips, many ready-to-paste examples, and three quick rewrites.

Quick answer

Use parenthesis only for one curved mark. In nearly every sentence you mean the pair - use parentheses. Replace "in parenthesis" with "in parentheses" and avoid spaces inside the marks.

parenthesis = singular (one mark)
parentheses = plural (the usual pair that opens and closes)
Most of the time, use parentheses

Core explanation: singular vs plural

Parenthesis refers to a single curved mark. Parentheses refers to both the opening and closing marks together. For asides, citations, phone numbers, math grouping and most uses, use parentheses.

Singular: The parenthesis is missing. (rare, when you truly mean one mark)
Plural: The parentheses are mismatched. (common)

Wrong: Please enclose your name and address in parenthesis.
Right: Please enclose your name and address in parentheses.

Grammar: agreement and common verb mistakes

Treat parenthesis like any singular noun when you mean one mark and parentheses like a plural noun. That affects verbs and determiners: use is/are, this/these accordingly.

Correct singular: The missing parenthesis is on line 3. (only if one mark)
Correct plural: The parentheses are out of order.

Wrong: The parenthesis are mismatched in the sentence.
Right: The parentheses are mismatched in the sentence.
Right (singular): The parenthesis is on line 3. (when referring to one mark)

Spacing and punctuation

Do not add spaces inside parentheses: write (example), not ( example ). Put sentence punctuation outside the closing parenthesis unless the parenthetical is a complete sentence.

If the parenthetical is a full sentence, capitalize it and include the period inside: (This is a full sentence.) If the punctuation applies to the whole sentence, place it after the closing parenthesis.

Correct: She bought apples (3), bananas (2), and pears (5).
Wrong: She bought apples ( 3 ), bananas ( 2 ), and pears ( 5 ).
Correct full-sentence parenthetical: He paused. (It had been a long day.)
Wrong spacing: There are ( 3 ) items in the box.
Right spacing: There are (3) items in the box.

Hyphenation and attributive use

Use parentheses as a noun. When you turn the word into an adjective before another noun, hyphenate only if needed for clarity: parentheses-free layout is clear; parentheses placement needs no hyphen.

Prefer: parentheses placement, parentheses usage
Hyphenate when ambiguity arises: parentheses-free layout

Wrong: We need a parenthesis-free layout for the document.
Right: We need a parentheses-free layout for the document.
Work - Right: Check parentheses placement in the procedure.

Memory tricks and quick checks

If you can't picture a single curved mark, use parentheses. The word ends in -s to indicate several - the pair. Search your draft for "in parenthesis" and change it to "in parentheses" unless you truly mean a single mark.

Quick check: search for "in parenthesis" and change to "in parentheses" when appropriate.
If "in bracket" feels wrong, "in parenthesis" is likely wrong too - use "in parentheses" or "in brackets" as needed.

Try your own sentence

Test the whole sentence instead of just the phrase. Context usually makes the right choice clear.

Examples: ready-to-use wrong/right pairs (work, school, casual)

Each wrong example shows a typical mistake; each right example is corrected and ready to paste into emails, reports, essays or posts.

Work - Wrong: Please enclose your signature in parenthesis before you return the contract.
Work - Right: Please enclose your signature in parentheses before you return the contract.
Work - Wrong: For line 12, put the citation in parenthesis next to the clause.
Work - Right: For line 12, put the citation in parentheses next to the clause.
Work - Wrong: Attach the reviewer comments in parenthesis at the bottom of the document.
Work - Right: Attach the reviewer comments in parentheses at the bottom of the document.
School - Wrong: Use a parenthesis to group the expression (2x + 3)(x - 1) when you expand it.
School - Right: Use parentheses to group the expression (2x + 3)(x - 1) when you expand it.
School - Wrong: Refer to the definition in parenthesis on page 4.
School - Right: Refer to the definition in parentheses on page 4.
School - Wrong: Place the citation in parenthesis after the sentence in APA format.
School - Right: Place the citation in parentheses after the sentence in APA format.
Casual - Wrong: I put my phone number in parenthesis at the end of the post.
Casual - Right: I put my phone number in parentheses at the end of the post.
Casual - Wrong: Don't write your DOB in parenthesis on the form.
Casual - Right: Don't write your DOB in parentheses on the form.
Casual - Wrong: She added her handle (insta) in parenthesis in the bio.
Casual - Right: She added her handle (Insta) in parentheses in the bio.

Rewrite help: three paste-ready fixes

Three quick patterns: swap the word, move the aside into a parenthetical, or integrate the aside so parentheses aren't needed.

Template A - simple swap: change "in parenthesis" → "in parentheses".
Template B - move aside: Main clause. (Aside.)
Template C - integrate: Turn the aside into a clause with commas or dashes.

Rewrite:
Wrong: Please send your reference (if available) in parenthesis.
Rewrite: Please send your reference in parentheses if available.
Rewrite:
Wrong: We require the dates (start and end) in parenthesis next to each task.
Rewrite: We require the start and end dates in parentheses next to each task.
Rewrite:
Wrong: My phone (work) is 555-0199-don't put it in parenthesis.
Rewrite: My work phone is 555-0199 (please don't include it in parentheses).

Similar mistakes to watch for

Writers who mix up parenthesis/parentheses often confuse other bracket names, spacing, or plural agreement. Learn the symbols and their typical uses to avoid slips.

Parentheses = ( ), square brackets = [ ], braces = { }.
Don't write "in bracket" - prefer "in brackets" or "in square brackets" as needed.
Watch related agreement errors (for example, "the datum are" vs "the data are").

Wrong: Please put the translator's note in bracket.
Right: Please put the translator's note in square brackets.
School - Usage: Math: Use parentheses ( ), square brackets [ ] and braces { } for nested groupings.

Real usage and tone: formal vs academic vs casual

Formal and academic prose: use parentheses sparingly. Short asides often read better with commas or dashes. In math and code, parentheses are precise and frequent.

Casual writing: parentheses are fine for aside comments, but still use the plural form - parentheses.

Formal: Prefer integrated clauses - commas or dashes rather than excess parentheses.
Academic/math: Use parentheses for citations and grouping; keep plural agreement.
Casual: Parentheses are acceptable, just avoid misusing "parenthesis".

Work - Usage: Formal: The study (n = 120) demonstrates a consistent effect.
School - Usage: Academic: Compute f(x) = (2x + 3)(x - 1) using parentheses for grouping.
Casual - Usage: Casual: I finally booked the flight (hallelujah!), so I'm coming.

FAQ

Is it parenthesis or parentheses?

Parenthesis is singular (one mark). Parentheses is plural (the pair). In everyday writing, use parentheses.

Should I write "in parenthesis" or "in parentheses"?

"In parentheses" is correct when you mean the pair around aside material. "In parenthesis" is a frequent error.

Is there ever a space just inside parentheses?

No. Do not add a space immediately after "(" or before ")": write (example), not ( example ).

When should I use square brackets instead of parentheses?

Use square brackets [ ] for editorial insertions into quoted material, for parentheticals inside parentheses, or when a style guide requires them.

Can "parenthesis" be correct in normal writing?

Only when you truly mean a single curved mark or when discussing typography. For enclosing text, use parentheses (the pair).

One quick check you can do now

Search your draft for "in parenthesis" and change it to "in parentheses" unless you mean one specific mark. If unsure, paste a sentence into a grammar checker and review the suggestion for parenthesis/parentheses and spacing.