Between independent clauses when a coordinating conjunction is omitted
Most commonly, the semicolon is used between two independent clauses (i.e., clauses that could stand alone as separate sentences) when a coordinating conjunction (for, and, nor, but, or, yet, so) is omitted.
Example
The upperclassmen are permitted off-campus lunch; the underclassmen must remain on campus.
The example above could be recast with the conjunction but, in which case a comma, rather than a semicolon, would be required.
Example
The upperclassmen are permitted off-campus lunch, but the underclassmen must remain on campus.
Technically, the semicolon could be replaced with a period, since each independent clause is a complete sentence. The semicolon, however, emphasizes the connection between the two clauses.
Note: When the second clause expands on or explains the first, the colon is the better mark.
Between independent clauses linked by a transitional expression
The semicolon is also used between two independent clauses linked by a transitional expression (e.g., accordingly, consequently, for example, nevertheless, so, thus).
Examples
Heavy snow continues to fall at the airport; consequently, all flights have been grounded.
Hyperinflation makes it extremely difficult to keep track of prices; thus a quart of milk might cost $10 in the morning and $200 in the afternoon.
In lists with internal commas
The semicolon can also be used in lists with internal commas. In this usage, the semicolon acts as a sort of super-comma.
Example
The new store will have groceries on the lower level; luggage, housewares, and electronics on the ground floor; men’s and women’s clothing on the second floor; and books, music, and stationery on the third floor.
In elliptical constructions
When combined with a comma, the semicolon can be used in elliptical constructions. In this case, the comma serves as an ellipsis, eliminating the need to repeat an understood portion of the initial clause.
Examples
In 1992, Starbucks had fewer than 200 stores; in 2002, almost 20,000.
Some people brought food; others, clothing; yet others, merely a willingness to help.