How do you punctuate nonessential information, and why must the marks at both ends match?
Nonessential elements and supplements: setting off nonessential information with a matched pair of commas, dashes or parentheses, distinguishing essential from nonessential, and keeping the opening and closing marks consistent.
A focused answer to the Digital SAT supplements skill: setting off nonessential information with a matched pair of commas, dashes or parentheses, distinguishing essential (no commas) from nonessential (paired commas), and the rule that the two enclosing marks must match.
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What this skill is asking
A supplement is a piece of nonessential information added to a sentence, and it must be set off by a matched pair of marks. On the Digital SAT, the College Board (Standard English Conventions domain) tests whether you can distinguish essential from nonessential information and punctuate the supplement consistently, with two commas, two dashes, or two parentheses, never a mismatched pair. The most testable rule is consistency: the mark that opens the supplement must be the same kind that closes it.
Essential versus nonessential
The first decision is whether the element is essential (needed to identify the noun) or nonessential (extra detail). The removal test settles it.
For example, in "My brother who lives in Tokyo is visiting," the clause "who lives in Tokyo" is essential if you have several brothers (it identifies which one), so it takes no commas. In "My only brother, who lives in Tokyo, is visiting," the brother is already identified, so the clause is nonessential and takes a pair of commas.
Consistency is the rule the SAT tests most
Even when you correctly judge an element as nonessential, the SAT most often tests whether the two enclosing marks match. A supplement cannot open with a comma and close with a dash, or open with a mark and close with nothing.
A single comma in the middle is almost always wrong
A useful shortcut: a single comma dropped into the middle of a sentence, with no partner, is almost always an error, either an unmatched supplement or a comma splitting a subject from its verb. When you see one stray comma, check whether it should be part of a matched pair (nonessential element) or removed entirely (essential element, or a wrongly split subject and verb). This is the same labelling discipline used across the boundaries skills, from sentence boundaries and clauses onward: decide the role of the element, then apply the matching rule. In practice, the fastest route through a supplement question is to find the opening mark in the underlined portion, then look for its partner at the other end of the inserted information: if the partner is missing or is a different kind of mark, that choice is wrong before you even weigh its meaning.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Digital SAT R&W (style)1 marksWhich choice correctly sets off the nonessential phrase? 'The author's first novel ____ has been translated into twelve languages.' (A) , a quiet success, (B) , a quiet success (C) a quiet success, (D) a quiet successShow worked answer →
The correct answer is (A), a matched pair of commas.
"A quiet success" is nonessential information describing the novel; the sentence is complete without it ("The author's first novel has been translated..."). Nonessential information is set off with a pair of commas, one before and one after. Choice (B) has only an opening comma; (C) has only a closing comma; (D) has none. The supplement needs commas on both ends.
Digital SAT R&W (style)1 marksWhich choice is correct? 'The scientist ____ presented her findings at the conference.' (A) , who won the prize last year (B) , who won the prize last year, (C) who won the prize last year, (D) who won the prize last yearShow worked answer →
The correct answer is (D), no commas.
Here "who won the prize last year" is presented without commas, treating it as essential to identifying the scientist; with no commas on either side, choice (D) is internally consistent and leaves the clause as part of the main sentence. Choices (A) and (C) add a single, unmatched comma, which is wrong; (B) sets the clause off as nonessential with a pair of commas, which would also be acceptable only if matched, but among these options the consistently punctuated choice is (D). The key rule is consistency: either a matched pair or none, never one comma.
Related dot points
- Sentence boundaries and clauses: distinguishing independent clauses, dependent clauses and phrases, and choosing the punctuation that correctly joins or separates them on a Digital SAT boundaries question.
A focused answer to the Digital SAT boundaries skill of recognising independent clauses, dependent clauses and phrases, then applying the punctuation rules that join or separate them, the foundation for every boundaries question.
- Commas and coordination: using commas correctly with coordinating conjunctions, in lists, after introductory elements, and not between a subject and its verb, on Digital SAT boundaries questions.
A focused answer to the Digital SAT comma rules: the comma plus coordinating conjunction for two independent clauses, commas in a series, commas after introductory elements, and the rule against commas that wrongly split a subject from its verb.
- Semicolons, colons and dashes: using a semicolon between two independent clauses, a colon after a complete clause to introduce, and dashes to set off or emphasise, on Digital SAT boundaries questions.
A focused answer to the Digital SAT rules for semicolons (between two independent clauses), colons (after a complete clause to introduce a list or explanation), and dashes (to set off or emphasise), with the complete-clause test that decides each.
- Avoiding comma splices and run-ons: recognising two independent clauses wrongly joined by a comma or by nothing, and choosing the correct fix (period, semicolon, comma plus conjunction, or subordination).
A focused answer to the Digital SAT skill of spotting and fixing comma splices and run-on sentences: recognising two independent clauses, applying the four valid fixes, and watching for conjunctive adverbs like 'however' that do not fix a splice on their own.
Sources & how we know this
- Reading and Writing: Content Domains and Skills — College Board (2024)
- Digital SAT Sample Questions — College Board (2024)