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Boundaries (punctuation, sentence structure)
What this skill is
Boundaries questions ask you to choose the punctuation that correctly connects (or separates) parts of a sentence. The stem is always "Which choice completes the text so that it conforms to the conventions of Standard English?" The choices are usually the same words with different punctuation: commas, semicolons, colons, dashes, periods, or nothing at all.
Key ideas
- An independent clause (IC) has a subject and verb and can stand alone as a sentence: "The museum opened in 1890."
- A dependent clause cannot stand alone: "because the museum opened," "which the museum displays."
- Two independent clauses can't be joined with just a comma (a comma splice) or with nothing (a run-on).
- A nonessential element (extra information) must be set off with matching marks on both sides: two commas, two dashes, or two parentheses.
- Don't put punctuation between a subject and its verb, between a verb and its object, or before a list that the sentence's grammar leads straight into.
Formulas and rules
| Joining... | Correct | Incorrect |
|---|---|---|
| IC + IC | IC**.** IC · IC**;** IC · IC**,** and IC · IC**:** IC (if the second explains the first) | IC**,** IC · IC IC |
| IC + dependent clause | IC dependent · IC**,** which... | IC**;** dependent · IC**.** Which... |
| IC + list or explanation | IC**:** list | subject verb**:** list ("include: X") |
| Nonessential element | , X , · — X — | , X — (mismatched) |
| Subject + verb | no punctuation | subject**,** verb |
Key facts:
- Semicolon = period. Both sides must be independent clauses.
- Colon must follow a complete independent clause. What comes after it explains, lists, or illustrates.
- Conjunctive adverbs ("however," "therefore," "for example") are not conjunctions. Between two ICs they need a semicolon or period before them: "...rain**;** however, ..."
Worked example 1 (easy)
The orchestra rehearsed for six weeks before the ______ the performance sold out within an hour.
A) concert, B) concert C) concert; D) concert, and,
- Split the sentence: "The orchestra rehearsed for six weeks before the concert" is an IC. "The performance sold out within an hour" is also an IC.
- A is a comma splice. B is a run-on.
- D adds a comma after "and," which is unnecessary.
- C: a semicolon correctly joins two ICs. Answer: C.
Worked example 2 (SAT-level)
The town's oldest building, a stone mill constructed in ______ still uses its original water wheel to grind grain for local bakeries.
A) 1782; B) 1782 C) 1782— D) 1782,
- Find the core sentence: "The town's oldest building ... still uses its original water wheel."
- "a stone mill constructed in 1782" is extra information. It opens with a comma after "building," so it must close with a comma.
- A: a semicolon would cut the subject off from its verb.
- B: no closing mark leaves the nonessential element unbalanced.
- C: a dash doesn't match the opening comma.
- D closes the pair. Answer: D.
Common traps
- Mismatched pairs. Opening with a comma and closing with a dash (or vice versa).
- Comma before "however" between two ICs. "...late, however, the..." is a splice. Use "late; however, the..."
- Colon after an incomplete clause. "The recipe needs: flour" is wrong. "The recipe needs three things: flour..." is right.
- Semicolon before a dependent clause. "...the plant; which grows..." is wrong; "which" can't start an independent clause.
- Extra commas. A comma between a subject and its verb ("The result of the study, was clear") is always wrong.