Using a comma before and after "but" is one of those punctuation questions that trips up even careful writers. The word "but" can function as a coordinating conjunction, an adverb, or a preposition, and which role it plays decides whether a comma belongs next to it. The good news: the rule is learnable in about five minutes.
The main rule: comma before "but" when joining two independent clauses
When "but" joins two independent clauses (two groups of words that could each stand alone as a sentence), put a comma before it. That's the core rule, and it covers most situations you'll encounter in business writing.
Consider these two sentences:
- The report was due on Friday, but Sarah didn't submit it until Monday.
- We quoted the project at $15,000, but the client negotiated it down to $12,000.
In both cases, each half of the sentence could stand alone. "The report was due on Friday" is a complete thought. "Sarah didn't submit it until Monday" is also a complete thought. The comma before "but" signals that two equal-weight clauses are being joined and contrasted.
Remove the comma when "but" connects a clause to something that can't stand alone. "He worked quickly but carefully" has no comma because "carefully" isn't an independent clause; it's a single adverb. Same with "The proposal was short but effective." Short and punchy sentences like these read better without the pause a comma creates.
When you should not use a comma before "but"
Three situations call for no comma before "but."
First, when "but" joins two single words or short phrases: tired but determined, simple but powerful, old but functional. No comma.
Second, when "but" connects a verb to a dependent clause that can't stand alone: She didn't finish the draft but promised to send it by morning. The second half, "promised to send it by morning," has no subject of its own, so it isn't independent. Drop the comma.
Third, when "not only … but also" frames a parallel construction: The plan reduces costs but also improves morale. Some style guides do permit a comma before "but also" in longer sentences, but for most business writing, leaving it out reads more cleanly.
A comma after "but": when does it apply?
A comma after "but" is much rarer, and it's often a sign of a different grammatical move. You'd place one after "but" when the word is followed by an interrupting phrase or a parenthetical that needs to be set off.
Example: But, as the legal team noted, the clause had already expired.
Here, "as the legal team noted" is a mid-sentence aside. It gets commas on both sides, and "but" happens to be the word that sits just before it. The comma isn't strictly after "but" because of "but" itself; it's there because the parenthetical phrase requires it.
Another example: But, frankly, the numbers don't support that conclusion.
Same logic. "Frankly" is a sentence adverb treated as a parenthetical, so it takes commas on both sides. The "but" precedes it and gets a comma attached to its right side as a result.
Writers who understand using commas with introductory clauses and phrases will recognize that pattern immediately. Whenever a word or phrase interrupts the flow of a sentence, commas mark both entry and exit.
Starting a sentence with "but"
You can start a sentence with "but." The old schoolroom rule against it was never a real grammar rule. Plenty of professional writers and major style guides endorse it. When you do start with "but," no comma follows it unless there's a parenthetical directly after it, as shown above.
Wrong: But, the client changed the brief again. (No parenthetical; no comma needed.)
Right: But the client changed the brief again.
If you're not yet sure whether starting a sentence with a conjunction is acceptable in your context, the post "But" it's okay: an introduction to starting sentences with conjunctions covers the full picture.
The "not … but" construction
A specific pattern worth knowing: not X but Y. This construction sets up a contrast between what something is not and what it is. It doesn't take a comma.
Example: The decision was made not by the board but by the CEO alone.
Adding a comma before "but" here would split the contrast in a way that disrupts reading. Keep it tight.
Quick summary to apply at your desk
The practical test is fast. Before placing a comma, ask: can the words after "but" stand alone as a complete sentence? If yes, use a comma. If no, don't. For a comma after "but," ask: is there an interrupting phrase directly following it? If yes, the phrase gets commas on both sides and "but" picks up the first one.
Punctuation around conjunctions follows this kind of structural logic throughout English. Writers who learn to spot an independent clause rarely second-guess themselves on this question again. And if you find yourself puzzling over related decisions, checking whether a modifier is hyphenated or not uses a similar "can it stand alone?" test that makes the logic click.
Write the sentence. Read both halves. If either half falls apart on its own, skip the comma before "but" and keep the sentence moving.