The key ideas
- A complete sentence has an independent subject-verb unit and a complete thought.
- A period, semicolon, or comma plus a coordinating conjunction can join two independent clauses.
- Commas and dashes can enclose nonessential information, but the pair must be complete.
Identify the clauses before choosing punctuation
An independent clause contains a subject and a verb and expresses a complete thought. A dependent clause may have a subject and verb but cannot stand alone, often because it starts with a word such as although or because.
Two independent clauses can be separated by a period or joined with a semicolon. A comma plus a coordinating conjunction such as and or but can also join them. A comma alone creates a comma splice.
Check colons and paired punctuation
A colon follows a complete clause and introduces an explanation, example, or list. Do not place a colon directly between a verb and its object. Nonessential information can be enclosed by matching commas or dashes; temporarily remove that information to check that the main sentence still works.
Worked example
The archive closed at six ___ the researchers returned the next morning.
- Both sides are independent clauses.
- A semicolon can join two closely related independent clauses without a conjunction.
Answer: The archive closed at six; the researchers returned the next morning.
A common mistake to avoid
Using a comma alone between two independent clauses because the pause sounds natural.
Your turn: try a free question
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Curriculum and sources
The key ideas and worked example come from PrepSAT's reviewed lesson curriculum. These College Board resources define the SAT scope; PrepSAT is an independent practice resource and is not affiliated with College Board.