When a conjunction (words like and, but and or) is used to merge two independent clauses into one sentence, it is possible to use a semicolon before the conjunction to outrank any commas in the clause. (This practice is acceptable, but it is considered outdated these days. However, if you think a semicolon makes your sentence clearer, you can use one.)
Semicolon before a ConjunctionWhen a sentence made up of two independent clauses contains commas, it is possible to use a semicolon before a conjunction which joins the two indepedent clauses to outrank any commas in those clauses.
semicolon used before but to outrank the other commas (correct usage)
IT'S OLD FASHIONED
Many people consider it old fashioned to use a semicolon before a conjunction these days.
Words like and, but and or are called conjunctions. Sometimes (as in the examples below), they join two sentences together to form one. When conjunctions are used in this way, they should be preceded by a comma. In this role, they are called co-ordinate conjunctions.
Sentence 2: She loves watching golf.)
(but merges the two sentences – co-ordinate conjunction – comma required)
Note: The word co-ordinate just means of equal rank. In these sentences, both halves are considered to be the same rank (i.e., they are both full sentences).
What are conjunctions?
Run-on error with a comma
Conjunctions and commas
Conjunctions and semicolons
Using semicolons before transitional phrases (e.g. however)
Using semicolons in lists
Using semicolons to extend a sentence