The semicolon is one of the most frequently misused and misunderstood marks of punctuation. The semicolon may be used correctly in only two ways:
(1) Use the semicolon to connect two independent clauses not linked by a coordinating conjunction.
(2) Use the semicolon to separate coordinate elements that are internally punctuated by a comma.
Rule 1 covers the most common use of the semicolon. It applies to instances such as these:
> Two independent clauses are connected by a transitional word.
I need to write better; however, grammar bores me.
> Two independent clauses are connected by a transitional phrase.
Your essay has some merit; at least, it is clear and concise.
> Two complete ideas are balanced in a "not only . . .but also" sentence.
This class is not only useful; it is also interesting.*
> Two related independent clauses are juxtaposed without a coordinating conjunction.
Some people prefer to pay cash; others prefer credit cards.
In all of the above examples, a complete thought appears on each side of the semicolon. Thus, for this principal use of the semicolon, it has the same strength as a period. Unlike the period, however, it is not followed by a capital letter (unless, of course, the next word is capitalized for some other reason – e.g., it is a proper noun).
Rule 2 refers chiefly to the use of the semicolon in a series that is itself internally punctuated by commas. Here are two examples:
The officers elected at the meeting were Ray Shapiro, president; Mary Shafer, vice president; June Atkins, treasurer; and Al Sharp, secretary.
This report contains an executive summary, which includes our recommendations; a detailed findings section, which presents pertinent data; and an Appendix, which describes our methodology.
Semicolons never connect parts of unequal grammatical rank. Thus, it is incorrect to have a dependent clause or a phrase on one side of the semicolon and an independent clause on the other side. It is not correct to write: "Although Mr. Smith, the president of the company, holds a doctorate degree; he is known for his earthy language." The "although" clause is dependent; the other clause is independent. The commas around "the president of the company" do not justify the use of a semicolon. This is not a series with internal punctuation. The proper punctuation after degree is a comma.
*When the construction is "not only . . . but also," only a comma is needed: "This is not only a user-friendly program, but it is also inexpensive."