4.2 Boundaries: Between Sentences
Key Takeaways
- The Between Sentences sub-point tests one thing: using contextually appropriate punctuation to mark where one sentence ends and the next begins.
- When independent clauses sit on both sides of a blank, the legal separators are a period, a semicolon, a colon (only if the second clause explains the first), or a comma plus a coordinating conjunction.
- A comma alone between two independent clauses is a comma splice, and no punctuation at all is a run-on — both are always wrong.
- Conjunctive adverbs (however, therefore, moreover, nevertheless) do not join clauses: they need a semicolon or period before them and usually a comma after.
- Coordinating conjunctions are the seven FANBOYS words — for, and, nor, but, or, yet, so — and only those seven can join two independent clauses after a comma.
The Question This Sub-Point Asks
College Board's description of Between Sentences is short: students will use contextually appropriate punctuation to properly mark the end of a sentence. In practice, every item in this sub-point puts a blank between two independent clauses and asks which mark belongs there.
So the diagnostic from section 4.1 does most of the work. Confirm that both sides are independent clauses. Once you have, only four separators are legal.
The Four Legal Separators
| Separator | When to use it | Example |
|---|---|---|
| Period (.) | Always available; the safest join for two fully independent ideas | "The bridge is old. Engineers inspect it every month." |
| Semicolon (;) | Two closely related independent clauses, no conjunction | "The bridge is old; engineers inspect it every month." |
| Colon (:) | Only when the second clause explains, specifies, or delivers what the first promised | "The inspection found one problem: the deck had corroded." |
| Comma + coordinating conjunction | Two independent clauses linked by a FANBOYS word | "The bridge is old, but engineers inspect it monthly." |
A period and a semicolon are functionally interchangeable on the SAT when both sides are independent and no conjunction is present. If both appear as choices in a way that makes them equivalent, re-read — something else is separating them, usually the presence or absence of a conjunction.
FANBOYS: The Only Seven
Only seven words are coordinating conjunctions, and they are the only words that can join two independent clauses after a comma:
For · And · Nor · But · Or · Yet · So
"The trial failed, but the team published the data." ✓
Anything outside those seven is not a coordinating conjunction, no matter how connective it feels.
Conjunctive Adverbs Are Not Conjunctions
This is the single most-tested distinction in the sub-point. Words like however, therefore, moreover, nevertheless, consequently, furthermore, instead, thus, otherwise are conjunctive adverbs. They signal a logical relationship but have no grammatical power to join clauses.
- ✗ "The report was submitted on time, however, the committee did not read it." → comma splice.
- ✓ "The report was submitted on time**; however,** the committee did not read it."
- ✓ "The report was submitted on time**. However,** the committee did not read it."
The rule: semicolon or period before a conjunctive adverb, comma after it. The exam offers the reversed version ("on time, however; the committee") as a distractor almost every time.
A useful proof: you can move a conjunctive adverb inside its clause without breaking the sentence — "The committee, however, did not read it." You cannot do that with but. That mobility is exactly why it cannot function as a joint.
When the Colon Is Right
A colon between two independent clauses is legal only when the second delivers on a promise made by the first — it explains, specifies, or names what the first clause set up.
- ✓ "The committee reached one conclusion: the bridge had to close."
- ✗ "The committee reached one conclusion: and the bridge closed." (The colon does not take a conjunction after it.)
- ✗ "The bridge is old: engineers inspect it monthly." (The second clause does not explain the first; it adds a related fact. Use a semicolon.)
The colon also has an absolute requirement that applies everywhere it appears: whatever precedes a colon must be a complete independent clause. That rule is developed further in section 4.3, where colons introduce lists and appositives rather than clauses.
The Two Errors That Are Always Wrong
| Error | Looks like | Why it fails |
|---|---|---|
| Comma splice | "The lab was empty, the experiment ran anyway." | A comma has no power to join independent clauses without a FANBOYS word |
| Run-on / fused | "The lab was empty the experiment ran anyway." | No boundary marker at all |
If you have confirmed two independent clauses, you can eliminate any choice offering a bare comma or nothing, immediately, without reading the rest of the sentence. That is often two of the four choices gone in five seconds.
Worked Examples
Example 1. "The telescope detected a faint signal from Proxima b ______ the planet orbits the closest star to the Sun."
Left side: "The telescope detected a faint signal from Proxima b" — IC. Right side: "the planet orbits the closest star to the Sun" — IC. Two ICs, and the second explains what Proxima b is. A semicolon works; a colon also works and is arguably better because the second clause specifies the first. A bare comma is a splice; nothing is a run-on.
Example 2. "Enrollment rose for six consecutive years ______ the district still closed two schools."
Both sides are ICs, and the second contradicts the first. Comma + but is the natural fix. A semicolon would also be grammatical, so if both appear, check whether the choices supply the conjunction — a semicolon plus "but" ("years; but the district") is wrong, because a semicolon already does the joining.
Section Takeaways
- Confirm two independent clauses first; then only four separators are legal.
- FANBOYS are the only coordinating conjunctions, and they need a comma before them when joining ICs.
- Conjunctive adverbs take a semicolon or period before and a comma after — never a bare comma before.
- A colon between clauses requires the second to explain or deliver on the first.
- Comma splices and run-ons are automatic eliminations once you have confirmed two ICs.
A passage reads: "The report was submitted on time ______ the committee did not read it until March." Which choice completes the text so that it conforms to the conventions of Standard English?
A passage reads: "Enrollment rose for six consecutive years ______ the district still closed two elementary schools." Which choice completes the text so that it conforms to the conventions of Standard English?
Which sentence correctly uses a colon between two independent clauses?
Two independent clauses sit on either side of a blank, with no conjunction supplied. Which of the following completions are grammatically legal? (Select all that apply)
Select all that apply