Semicolon

A semicolon (;) links two independent clauses — word groups that could each stand alone as complete sentences — into a single sentence without using a coordinating conjunction. "The experiment failed; the team started over" is correct because both halves are complete thoughts. The semicolon signals that the ideas are closely related, more tightly than a period would suggest.

The most important test is that a semicolon must have a complete sentence on both sides. "Although the experiment failed; the team started over" is wrong, because the first half is a dependent clause. A semicolon also should not appear before a coordinating conjunction like "and" or "but" joining two clauses — that job belongs to a comma. However, a semicolon does correctly precede conjunctive adverbs such as "however," "therefore," and "moreover" when they begin the second clause: "The data looked promising; however, the sample was too small."

The semicolon's second job is the "super comma": separating list items that already contain commas. "The tour stops in Portland, Oregon; Boise, Idaho; and Helena, Montana" stays readable because semicolons mark the boundaries between items.

Punctuation questions on the ACT, SAT, and CLT test semicolons heavily. The fastest strategy: check whether the words on each side of the semicolon could stand alone as sentences. If both can, the semicolon (or a period, or a comma plus a FANBOYS conjunction) is acceptable; if either cannot, the semicolon is wrong.