Boundaries

What this skill tests

College Board calls this skill Boundaries. A blank stands where punctuation belongs, and the four choices usually contain the same words with different marks between them. That is your signal to test structure, not sound. Punctuation makes clause and phrase boundaries visible, so identify the sentence structure before you choose the mark. Do not pick the version that sounds best when you read it in your head. Let the real relationship between the parts of the sentence decide.

Start with sentence structure

Work through these questions in order.

1. Could each side of the blank stand as a sentence on its own? If yes, you are joining two independent clauses (see the five legal ways below).
2. Does the sentence open with a dependent clause beginning with because, although, when, if, or since? If yes, a comma commonly separates that clause from the main clause that follows.
3. Does the blank sit around a relative clause introduced by that, which, who, whom, or whose? If yes, decide whether the clause is restrictive or supplementary, then punctuate accordingly.
4. Can a stretch of words be lifted out with the sentence still working, and without changing which person or thing is identified? If yes, it is a supplement and needs a matching pair of marks.
5. Do the words identify which one? If yes, they are essential and take no marks at all.

Independent and dependent clauses

An independent clause can stand alone as a complete sentence. It normally has a subject and a finite verb (an imperative is the usual exception, since its subject is understood). A dependent clause has a subject and a verb too, but a word at its front holds the thought open.

The traps let juvenile fish escape. (independent)
Because the traps let juvenile fish escape (dependent: because waits for a result)
Which the crew installs each spring (dependent: which attaches to a noun elsewhere)

Words that often open dependent clauses: because, since, as, although, though, while, whereas, if, unless, when, whenever, before, after, until, once, that, which, who, whom, whose, where. Judge by function, not by the first word alone. Who came? and That matters. are complete sentences, because there the words are doing a different job.

Five ways to join two independent clauses

• Period: The traps have escape panels. Small fish slip out.
• Semicolon: The traps have escape panels; small fish slip out.
• Comma plus a coordinating conjunction (for, and, nor, but, or, yet, so): The traps have escape panels, so small fish slip out.
• Colon, when the second part explains, specifies, or delivers what the first part set up: The result was clear: small fish slip out.
• Single dash, for emphatic elaboration after a complete clause.

Three constructions always fail. A comma splice (“The traps have escape panels, small fish slip out”) uses a comma, which is too weak. A fused sentence (“The traps have escape panels small fish slip out”) uses nothing at all. And a doubled join such as a semicolon plus and stacks two joiners where one does the job. For ordinary SAT clause-joining items, choose either a semicolon by itself or a comma plus a coordinating conjunction.

Two common traps deserve special mention. First, the colon and dash need a complete setup before them. Second, a long clause is not automatically independent, and a clause hidden under long modifiers is not automatically dependent. Strip the extras and look for the subject and finite verb on each side.

Conjunctive adverbs are not conjunctions

However, therefore, moreover, nevertheless, consequently, instead, furthermore, and similarly are conjunctive adverbs. None of them is a coordinating conjunction like but, so none can join two independent clauses on a comma alone.

I studied for the exam; however, I still worried about the results.

Use a semicolon (or a period) before the adverb and a comma after it. Wherever the adverb sits in the second clause, the sentence still needs a period or semicolon between the two clauses.

Colons and dashes: the complete-setup rule

Whatever stands before a colon must be able to end with a period on its own. What follows may be a clause, a phrase, or a list, but it must deliver what the setup announced.

• The team faced one problem: the battery could not hold a charge. Complete setup, complete delivery. Correct.
• The team needed: three instruments. Needed still wants its object. Broken.
• The study focused on: coastal erosion. The preposition on still wants its object. Broken.
• The species occurs in regions such as: wetlands and marshes. Such as already introduces the examples. A colon interrupts it.

Commas inside the setup do not block a colon: After considering cost, availability, and past performance, the committee reached its decision: the original contractor would continue the work.

A single dash after a complete clause can do a colon-like job (emphatic elaboration). A pair of dashes does a different job: it encloses a supplement inside a continuing sentence.

Enclosing supplementary material

When two marks enclose the same supplement, they must match: comma with comma, dash with dash, parenthesis with parenthesis. A comma opening and a dash closing (or the reverse) is a classic distractor. A supplement at the very end of a sentence needs only an opening mark, because the period closes it.

The escape panel, a rectangle of loose mesh, lets small fish slip out.
The escape panel — a rectangle of loose mesh — lets small fish slip out.

Always read past the blank. If a comma or dash appears later in the sentence, it may be the closing half of a pair, and the blank must supply the matching opener. Mixed marks are wrong only when they are meant to enclose the same supplement.

Restrictive versus supplementary information

Lift the enclosed words out and ask two questions, not one.

1. Is the sentence still grammatical?
2. Does it still point to the same person or thing and say the same thing about it?

If the answers are yes and yes, the words are a supplement and take a matching pair. If the sentence breaks, or if the reference changes, the words are essential and take no marks.

The students who completed the lab submitted reports. (essential: only some students)
The Mona Lisa, which hangs in the Louvre, draws millions of visitors. (supplement: the painting is already identified)

Notice that The students submitted reports is perfectly grammatical, yet the meaning changed from a subset to everyone. Removal alone is not the test. Likewise, that usually signals a restrictive clause and which a supplementary one in edited American English, but function decides first.

Appositives work the same way. Among several colleagues, my colleague Maya arrived early needs no commas, because Maya says which colleague. My only colleague, Maya, arrived early takes commas, because “my only colleague” already identifies one person.

Protect the sentence core

Students who learn that commas mark pauses put them wherever they would breathe. Do not. These structures hold together without punctuation:

• Subject and verb. The panels that the crew installs each spring are checked for wear. No comma before are, however long the gap.
• Verb and object or complement. The redesign reduced bycatch, damage, and cost. No comma after reduced.
• One subject, two predicates. The crew inspected the panels and repaired the frames. No comma before and. Compare Maya measured the samples, and Leo recorded the results, which has two subjects and two clauses.
• Dependent clause after the main clause. The pilot delayed takeoff because the runway lights had not been tested. The comma that follows an opening because clause does not automatically appear before a trailing one.

Introductory elements

An introductory phrase or clause is followed by a comma before the main clause. After reviewing the samples, the researchers revised the protocol. The opener cannot stand alone, so a semicolon or colon after it is wrong, and leaving the comma out runs the opener into the subject. Even a one-word opener can take a comma: Yesterday, the results arrived. Structure and convention decide, not length.

A phrase containing a verb form is not a clause. Having finished the trial and hoping the change would reduce bycatch have no finite main-clause predicate, so a semicolon in front of them fails.

Other boundary cases

• Complex lists. When items already contain commas, semicolons separate the large items: a quarry in Provence, France; a workshop in Turin, Italy; and a foundry in Leiden, the Netherlands. It is the same mark doing a different job from joining clauses.
• Dense proper-name sentences. Strip dates, titles, and institutions and find the skeleton: Dr. Vasquez published her findings. Everything else is modifier.
• Quotation marks. American convention puts commas and periods inside closing quotation marks. Decide the sentence's own boundary first, then place it.
• Questions. A direct question ends with a question mark. An indirect question (“The researchers asked whether the signal had changed”) is a statement and ends with a period.
• Inverted and existential sentences. The subject can follow the verb (Across the valley stand three towers), and an embedded clause can hold its own subject and verb (The proposal that the committee reviewed twice failed). Do not mistake an embedded pair for the main clause.

The fixed decision order

1. Identify what sits on each side of the blank: independent clause, dependent clause, phrase, or a piece integrated into the sentence.
2. Decide the blank's job: joining clauses, opening or closing a supplement, or marking no boundary at all.
3. Eliminate every choice that is structurally illegal for that job: a semicolon beside a phrase, a colon after an incomplete setup, mismatched marks.
4. Only among the survivors, let the logical relationship (contrast, result, example) help you choose.

If two choices survive all four steps, look again for a later mark in the sentence that is already doing half the job.

Quick review of hard cases

• Two complete sentences can hide under long modifiers. Test completeness after stripping the extras.
• A comma splice can hide behind formal vocabulary. Polished clauses are still clauses.
• Do not add a semicolon alongside a coordinating conjunction.
• However is not but.
• Length proves nothing: a six-word clause can be independent, and a thirty-word participial phrase can be dependent.
• A natural spoken pause is not a grammar category. Speech rhythm is unreliable.
• No punctuation is often correct when the blank sits inside one grammatical unit.

The method in five steps

1. Check what differs among the choices. If only punctuation changes, work from structure.
2. Cover the blank and read the left side. Could it end with a period?
3. Read the right side and ask the same.
4. If words sit in the middle, lift them out and test whether the sentence survives and still means the same thing.
5. Eliminate by rule, not by ear.

Worked example

The mesh panels ______ the crew installs before each season ______ checked for wear when the boat returns to harbour. The clause the crew installs before each season identifies which panels, so it is restrictive and takes no marks at either end. The correct choice is “that ... are,” with no commas. Adding a single comma before are would cut the subject from its verb, and adding commas around an identifying clause would treat it as removable.