Section 15.3 Sentence Boundaries
One of the most important punctuation decisions a writer makes is how to handle the boundary between two independent clauses. English provides four options, each signaling a different relationship between the ideas. Choosing among them is partly grammatical and partly rhetorical: the grammar determines what is legal, and the writer’s judgment determines what is effective.
1. Period: Full Separation.
A period creates the strongest boundary—two separate sentences, two separate ideas:
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The rain stopped. We went outside.
Use a period when the two ideas are related but do not need to be presented as a single thought.
2. Comma + Coordinating Conjunction: Compound Sentence.
A comma before a coordinating conjunction (and, but, or, nor, for, so, yet) joins two independent clauses into a compound sentence. The conjunction makes the logical relationship explicit:
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The sun set, and the stars appeared.
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She tried hard, but she didn’t succeed.
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He must study, or he will fail.
Important: The comma goes before the conjunction, not after. And the comma is required only when both sides are independent clauses. When a conjunction joins two phrases (not clauses), no comma is needed:
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She is talented and creative. (no comma—and joins two adjectives)
3. Semicolon: Close Relationship.
A semicolon signals that two independent clauses are closely related—so closely that the writer wants to present them as a single unit. It provides a boundary weaker than a period but stronger than a comma:
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The rain stopped; we went outside.
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She’s talented; however, she lacks experience.
Note the second example: when a conjunctive adverb (however, therefore, nevertheless, moreover) connects two independent clauses, use a semicolon before it and a comma after:
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He was exhausted; however, he continued working.
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The evidence is strong; therefore, we should proceed.
4. Colon: Anticipation.
A colon signals that what follows explains, illustrates, or completes what came before. The first clause sets up an expectation, and what follows the colon fulfills it:
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She had one goal: to finish her degree.
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The answer was clear: we needed more data.
The Comma Splice: What NOT to Do.
A comma splice occurs when two independent clauses are joined with only a comma—no conjunction, no semicolon:
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The rain stopped, we went outside.
This is an error in standard written English. Fix it by using one of the four options above:
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The rain stopped. We went outside. (period)
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The rain stopped, and we went outside. (comma + conjunction)
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The rain stopped; we went outside. (semicolon)
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The rain stopped: the clouds parted and the sun appeared. (colon, if anticipatory)
Comma After Introductory Adverbial Clauses.
When a dependent clause precedes the main clause, use a comma at the boundary:
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When the bell rang, students left.
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Because the roads were icy, we drove slowly.
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Although she was tired, she finished the report.
When the dependent clause follows the main clause, the comma is usually omitted:
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Students left when the bell rang. (no comma)
