Comma with the conjunction but

Question:

Do I need a comma before the conjuction but in the following sentence?

• The trainee wrote an in-depth report, but the supervisor requested a one-page summary.

Answer:

Yes, you need a comma. If two ideas are worded as independent clauses—meaning each can stand on its own as a sentence—they need to be separated by some mark of punctuation. In this case, you could have two separate sentences:

• The trainee wrote an in-depth report. The supervisor requested a one-page summary.

In your example, the two independent clauses are joined by the coordinating conjunction but, which must be preceded by a comma.

Interestingly, you may choose to omit the comma if the clauses are short or closely related. However, it is never incorrect to use a comma in such cases. For example:

• Life is short but art is long.

  OR

• Life is short, but art is long.