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From the OWL resource Commas
Commas vs. Semicolons in Compound Sentences
A group of words containing a subject and a verb and expressing a complete thought is called a sentence or an independent clause. Sometimes, an independent clause stands alone as a sentence, and sometimes two independent clauses are linked together into what is called a compound sentence. Depending on the circumstances, one of two different punctuation marks can be used between the independent clauses in a compound sentence: a comma or a semicolon. The choice is yours.
Comma (,)
Use a comma after the first independent clause when you link two independent clauses with one of the following coordinating conjunctions: and, but, for, or, nor, so, yet. For example:
Semicolon (;)
Use a semicolon when you link two independent clauses with no connecting words. For example:
You can also use a semicolon when you join two independent clauses together with one of the following conjunctive adverbs (adverbs that join independent clauses): however, moreover, therefore, consequently, otherwise, nevertheless, thus, etc. For example:
For more information about compound sentence patterns, see the Purdue OWL handout on Sentence Punctuation Patterns.