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The Essentials of Writing

The Hamilton College Style Sheet

Essentials of English Usage

Semicolon ("semi")

Independent clauses not joined by a coordinating conjunction must be separated by a semicolon.

As with that of comma splice, this definition is deceptively complicated.  Loosely translated, it means that if you don't use and, but, for, nor, or, so, or yet (the coordinating conjunctions) when joining two independent clauses, you have to use a semicolon. (A semicolon is analogous to a weak period; use it to join two sentences that work better together than alone.)

1. This class is mind-numbing, I think I'm going to pass out.
[Incorrect: comma splice. Use a semicolon instead: This class is mind-numbing; I think I'm going to pass out.

2. The students had sat up most of the night studying; however, they did not seem sleepier than usual.
[This is correct. Independent clauses here are not joined by a coordinating conjunction; hence, they are separated by a semicolon. However is a conjunctive adverb, and not to be confused with a coordinating conjunction.]

Some conjunctive adverbs:

however hence
then thus
moreover consequently
therefore furthermore
still in fact

For more information on punctuation, refer to Punctuation Patterns. See also Punctuation ("p").