Rule 101 of compound sentences is that a coordinate conjunction must be accompanied by a comma most of the time when joining independent clauses, yet I stumble across sentences all that time that seem to abuse this rule. Normally, it's a case of the subject not being present in the second clause, as in the following example:

"They usually start out as an abbreviation, but turn into acronyms when they become well known."
(taken from the Oxford Style Manual)

To me, this comma cannot be correct because the clause after the comma doesn't contain a subject and therefore cannot be independent (right?). As far as I'm concerned, it's not the 'Comma to set off a contrast rule because I can find plenty of other where 'and' is used and the subject's missing in the second clause. what's the deal here?

Thanks in Advance for any responses I may receive!

  • I would agree with you that in the sentence you mentioned the comma shouldn't be there... But seeing as how I have no idea why it shouldn't be there... all I can offer is a modest opion FWIW.
    – MegaMark
    Aug 14, 2015 at 2:49
  • @MegaMark The comma is optional (and I agree with aparente001 and OSM that it's preferable here). The mixing of 'They', 'an abbreviation' and 'acronyms' seems a poor advert for the style guide (if the quote is accurate). Aug 14, 2015 at 10:17
  • Is that an accurate quote? Could you possibly add a link? "They usually start out as an abbreviation, but turn into acronyms when they become well known." seems disagreeable Aug 14, 2015 at 10:25
  • @Edin Ashworth I can't find the page to cite for that sentence, but here's a similar example, again from the latest version of the 'New Oxford Style Manual': "A colon should not precede linking words or phrases in the introduction to a list, and should follow them only where they introduce a main clause" (pg.74, sec.4.5, chap. 4) Again, the comma persists between the compound predicate for some reason?
    – Sean
    Aug 14, 2015 at 22:02
  • I found the acronym quote in English Grammar for Dummies by Lesley J. Ward, Geraldine Woods. The google preview didn't have page numbers, sorry. Aug 15, 2015 at 19:13

2 Answers 2


The original sentence has a compound predicate. A compound sentence, by contrast, is two sentences joined by punctuation.

Most styles wouldn't use a comma to separate the parts of a compound predicate (or, put another way, to separate the subject of a sentence from any of its verbs). I would remove the comma, or else I'd keep it and then repeat the subject (or a pronoun of it): "They usually start out as an abbreviation, but they turn into acronyms when they become well known."

See, for example, 6.29 in the Chicago Manual of Style (16th edition): "A comma is not normally used between the parts of a compound predicate—that is, two or more verbs having the same subject, as distinct from two independent clauses (see 6.28)."

  • I completely agree with everything you've said, in terms of it being a compound predicate, but do you not think it's sometimes the writer's intention to mean it as a compound sentence, where the second subject ('they', in this example) has been elippted to avoid repetition? Also, what's everyone's thoughts on constructions when the second clause begins with 'and that' or 'so that', and it has a comma separating them? I come across these sentences all the time (all from esteemed writers) and cannot understand the rationale of the comma.
    – Sean
    Aug 14, 2015 at 16:13
  • @Sean - could you write a new question, please, with a couple of example sentences showing 'and that' and 'so that'? Aug 15, 2015 at 19:15
  • To answer your other question, might the writer have been thinking of it as a compound sentence -- I don't think so. I know I don't when I'm writing a sentence like the one you gave as an example. Aug 15, 2015 at 19:16

I think your problem is with the logic. What the compound sentences rule you're talking about is really saying is:

Rule 1: If a sentence is a compound sentence, then the conjunction should be accompanied by a comma (most of the time).

But the sentence you gave as an example isn't a compound sentence, i.e. it doesn't have two independent clauses, so the compound sentences rule doesn't apply.

I think the mistaken logical thinking you've been doing is to say, "Since this isn't a compound sentence, a comma must not be used." In other words, you've misinterpreted Rule 1 to mean:

Rule 2: If the conjunction is accompanied by a comma, then the sentence is a compound sentence.

You feel bothered, because you know your example sentence isn't a compound sentence!

However, Rule 2 does not follow logically from Rule 1.

Let's see what's going on in your example sentence.

They usually start out as an abbreviation, but turn into acronyms when they become well known.

Here we have a shared subject for the two main verbs. This will be clearer if I simplify the sentence and replace but with and:

They start out as A and turn into B.

The comma is optional here. I like it in your example sentence because the sentence is on the long side, and the comma gives us a nice small rest. Also, the comma accentuates the contrast.

  • Thanks for your response! So what explicit rule does this use of the comma fall under then (in terms of the syntax)? If you think about it, this type of construction creates ambiguity, because either it's a compound predicate, and there's a comma separating them for some reason, or it's a compound sentence, where the second subject has been ellipted--or implied--since it's easy to tell who the second clause is about.
    – Sean
    Aug 14, 2015 at 3:38
  • The comma isn't required, and I don't think there's a rule for this optional comma. But I did my best to provide my rationale for liking the comma in your example sentence. Aug 14, 2015 at 3:41
  • ^ updated it. I guess I'm just the kind of person that looks for concrete rules for every situation. Thanks for taking the time.
    – Sean
    Aug 14, 2015 at 3:46
  • This question (or ones like it) has been asked here before, but this answer is the best I've found on a cursory search. Aug 14, 2015 at 10:14
  • The rule is about not putting a comma in the middle of a two-part parallel construction, which is what you have in the example: They start out . . . but turn . . . ." There's a good explanation of it here: faculty.deanza.edu/flemingjohn/stories/storyReader$16
    – ewormuth
    Aug 14, 2015 at 16:31

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