A semicolon is used to separate two closely related sentences. In other words, you can remove a semicolon, replace it with a period, and remain grammatically correct. The same is not true with a comma.

Semicolons are not common in English. If in doubt, you probably shouldn’t use one.

Edit:

Another person answered while I was writing my answer and made the distinction of an independent clause. Where I said “two closely related sentences” above, it would, in fact, be more correct to say “two independent clauses”.

Good luck.

Brought to you by the Purdue University Online Writing Lab at http://owl.english.purdue.edu

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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:

I am going home, and I intend to stay there.

It rained heavily during the afternoon, but we managed to have our picnic anyway.

They couldn’t make it to the summit and back before dark, so they decided to camp for the night.

Semicolon (;)

Use a semicolon when you link two independent clauses with no connecting words. For example:

I am going home; I intend to stay there.

It rained heavily during the afternoon; we managed to have our picnic anyway.

They couldn’t make it to the summit and back before dark; they decided to camp for the night.

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:

I am going home; moreover, I intend to stay there.

It rained heavily during the afternoon; however, we managed to have our picnic anyway.

They couldn’t make it to the summit and back before dark; therefore, they decided to camp for the night.

For more information about compound sentence patterns, see the Purdue OWL handout, Sentence Punctuation Patterns at http://owl.english.purdue.edu/handouts/grammar/g_sentp.html.

You can try an exercise on using commas and semicolons in compound sentences at http://owl.english.purdue.edu/handouts/grammar/g_commacompEX1.html, with a separate answer key at http://owl.english.purdue.edu/handouts/grammar/g_commacompA1.html.

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