Q. I’ve been having a debate with a vendor regarding commas. What is the proper way to punctuate a compound sentence with an introductory clause that applies to both parts of the sentence? For example, “During percussion, tympany is a hollow sound over an air-filled structure and dullness is a thud-type sound over a solid structure.” Most style guides cover the need for a comma after an introductory phrase (unless it is very short and clear) and the need for a comma between the independent clauses of a compound sentence (unless they are very short and related), but they don’t provide specific guidance for both elements in one sentence. I have interpreted this omission to mean that the comma should generally be used in both places, after the introductory phrase and between the independent clauses, but my vendor is insisting that the comma isn’t needed between the independent clauses because the introductory phrase applies to both of them.

A. The introductory phrase is not the most important consideration, grammatically. In the sentence you provide, the comma before and is optional because the clauses are short; the lack of a comma also helps to show that the introductory phrase applies to both clauses. If your clauses were longer or more complex, however, they would need a comma to separate them even if they were both governed by the same introductory phrase.