How to solve SAT Boundaries questions on punctuation

Boundaries questions test punctuation at the edges of clauses and phrases. You decide where one sentence ends and the next begins, how to set off extra information, and when no punctuation belongs at all. The skill belongs to the Standard English Conventions domain, about 26% of the Reading and Writing section, or roughly 11 to 15 questions shared with Form, Structure, and Sense. Expect several Boundaries questions per test. Every one asks, "Which choice completes the text so that it conforms to the conventions of Standard English?" The choices usually contain the same words with different punctuation.

What the questions look like

• Joining clauses. The blank sits between two groups of words, and you choose among a period, a semicolon, a comma, a colon, or a comma plus a conjunction. These questions test run-ons and comma splices.
• Supplements. The sentence contains extra information, like a description or an aside, and you must close it with the same mark that opened it.
• No punctuation needed. The blank falls between a subject and its verb, a verb and its object, or a title and a name, where adding a comma would be wrong.

Step-by-step method

1. Compare the choices to see what changes. If only the punctuation differs, you are looking at a Boundaries question.
2. Find the subject and verb on each side of the blank, and decide whether each side could stand alone as a sentence. A group of words that can stand alone is an independent clause.
3. Apply the rule that fits:
   • Join two independent clauses with a period, a semicolon, or a comma plus a conjunction such as "and," "but," or "so."
   • A comma alone between two independent clauses is a comma splice, and it is always wrong on the SAT.
   • A semicolon needs an independent clause on both sides. The one exception is a list whose items already contain commas.
   • A colon needs an independent clause before it. What follows can be a list, a phrase, or a clause that explains or illustrates the first part.
   • Words like "however," "therefore," and "for example" can't join two clauses by themselves. Between two independent clauses, put a semicolon or period before them and a comma after.
   • Nonessential information is set off by a matching pair of marks: two commas, two dashes, or two parentheses.
   • Essential information takes no commas. In "The biologist Rachel Carson wrote about pesticides," the name tells you which biologist, so no commas go around it.
   • Never put a single comma between a subject and its verb, even when the subject is long.
4. If the blank might close a nonessential element, look back for the mark that opened it and match it.
5. Reread the full sentence with your choice in place.

Worked example

The Atacama Desert in northern Chile is one of the driest places on Earth ______ some weather stations there have gone years without recording measurable rain.

Which choice completes the text so that it conforms to the conventions of Standard English?

A) Earth,
B) Earth;
C) Earth
D) Earth, in fact,

Answer: B. "The Atacama Desert in northern Chile is one of the driest places on Earth" is an independent clause. So is "some weather stations there have gone years without recording measurable rain." A semicolon can join two independent clauses. Choice A is a comma splice. Choice C runs the clauses together with no punctuation. Choice D is still a comma splice, because "in fact" can't join clauses. "Earth; in fact," would have worked.

Here is a supplement question.

Mae Jemison, the first African American woman to travel into space ______ later founded a technology consulting company.

Which choice completes the text so that it conforms to the conventions of Standard English?

A) space,
B) space;
C) space
D) space:

Answer: A. The phrase "the first African American woman to travel into space" is extra information about Jemison, and it opens with a comma after her name. It has to close with a comma too. A semicolon or colon would cut the subject "Mae Jemison" off from its verb "founded," and no punctuation leaves the aside unclosed. A dash would also be wrong here, because a supplement that opens with a comma can't close with a dash.

Common traps

• The "however" splice. "The test was short, however, it was difficult" is a comma splice. Write "The test was short; however, it was difficult."
• Mismatched pairs. An aside that opens with a comma must close with a comma, and one that opens with a dash must close with a dash.
• A comma after a long subject. In "The researchers who studied the reef for a decade found new species," no comma belongs after "decade," because "found" is the verb for "researchers."
• A semicolon next to a fragment. "Although the trail was steep; we reached the top" is wrong because "Although the trail was steep" can't stand alone. Use a comma.
• Commas around essential names. "The novelist, Toni Morrison, won the Nobel Prize" treats her name as extra information, as if there were only one novelist. Write "The novelist Toni Morrison won the Nobel Prize."

Quick check

Cover the blank. Ask whether the words on the left could stand alone as a sentence, then ask the same about the words on the right. Those two answers decide most Boundaries questions.