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As far as I know, it’s a rule that a comma is needed before a coordinating conjunction that joins two independent clauses. But the use of a comma before a coordinating conjunction that joins two independent clauses in a compound-complex sentence seems to change the meaning of the whole sentence. Please compare the four sentences below.

  1. I swim every day and my friend plays tennis five times a week because exercise is good.
  2. I swim every day, and my friend plays tennis five times a week because exercise is good.
  3. I always feel fresh but John always feels somber whenever it rains.
  4. I always feel fresh, but John always feels somber whenever it rains.

I think that in 1. and 3. the context of the subordinate clause applies to both independent clauses, but in 2. and 4. it seems to apply to the independent clause that is adjacent to it.

Therefore, if we want the context of the subordinate clause to apply to both independent clauses, the rule that a comma is to be used before a coordinative conjunction that joins two independent clauses should be exempted in these cases. I have tried to find reliable references that state such exemption, but I rarely found one. The two references that I found (which were from small websites) said that a comma is not needed in such sentences with such context. I cannot find more references to confirm this opinion, so I’d like to ask for more opinions here regarding this.

N.B. I’ve checked the previous posts but couldn’t find the one that exactly addresses this question. The one that I found similar is “Comma before conjunctions in predicates containing two coordinate clauses”. But that one is about comma before conjunctions in predicates not comma before conjunctions that join independent clauses.

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The Limited Power of the Comma

Before a coordinating conjunction joining two independent clauses, a comma only signals the beginning of the second clause. Its presence or absence, however, cannot signal whether a subordinate clause at the end of the sentence is to be distributed across both independent clauses or only one: that’s a question of semantics, not grammar.

Like a pronoun, a modifier following what it is to modify grabs on to the first logically available target and tends to stay there:

I chose a pottery class, but Robert signed up for creative writing because it was the only class without a scheduling conflict.

There is no ambiguity in this sentence despite the position of the subordinate clause modifier. Just as it attaches itself to creative writing and is perfectly happy remaining there, the subordinated clause restricts itself to the second clause 1) because there is no cue to read further to parse the sentence and 2) such a reading is semantically blocked by the verb chose, i.e., between at least two alternatives, while Robert’s crowded schedule only allowed him to register for one class.

Your example sentences, however, have been intentionally constructed to introduce ambiguity by subordinating a statement in the gnomic present (“Exercise is good”) or in the habitual present (“Whenever it rains”), both signalling a motivating truth or condition universal enough to be parsed across both independent clauses. Yet you place both in final position, where the tendency is to restrict their meanings to the second clause only. The presence or absence of a comma cannot resolve the ambiguity because it is a semantic one you’ve built into the sentence in the first place.

Topicalization

Consider these two pairs:

I swim every day because exercise is good.
Because exercise is good, I swim every day.

My friend plays tennis five times a week because exercise is good.
Because exercise is good, my friend plays tennis five times a week.

The inversion in italics gives the cause for the action slightly more topicality than the standard order in roman, which a writer might choose for a smooth transition to the next sentence or merely to vary sentence structure for better style.

If you wish to combine the first of each pair using a coordinating conjunction, however, the topicality becomes far greater because it is a cause common to both. Otherwise, why would you utter the sentence in the first place? The usual way to topicalize the cause is to place it at the beginning, just as one does for each person alone:

Because exercise is good, I swim every day, and my friend plays tennis five times a week.

or keep the standard order for the first clause only and let ellipsis take care of the rest:

I swim every day because exercise is good, and my friend plays tennis five times a week [because exercise is good].

or in some fashion reinforce the universal truth that exercise is good:

I swim every day, and my friend plays tennis five times a week because, as people always say, exercise is good.

Ambiguity resolved, but commas have little to do with it.

Your second example is similarly resolved by placing the condition first or by maintaining normal word order for the first clause:

Whenever it rains, I always feel fresh, but John feels somber.

I always feel fresh whenever it rains, but John feels somber.

Semantic ambiguity can only be resolved by a semantic solution, perhaps aided by comma placement, but don’t demand that a comma do all the work by itself.

  • Thank you very much. I think I understand what you explained. But I still wonder. Isn't it true that a comma signifies a slight pause in the train of speaking or the train of thought? If so, the comma will have the effect of letting the subordinate clause attach more to the independent clause that is adjacent to it (like sentence 2. and 4. in my question above). If so, wouldn't it be better not to put comma before the coordinate conjunction to avoid the unintended effect? – user287279 Apr 10 '18 at 15:58
  • Here’s the link to the website that I referred to in my question: thepunctuationguide.com/comma.html#compound-complexsentences And here is its recommendation: Rule: When a sentence begins with a dependent clause that applies to two independent clauses that follow, insert a comma after the dependent clause, but do not insert a comma between the independent clauses. If we want this business to work, you need to find suppliers and I need to find buyers. – user287279 Apr 10 '18 at 16:04
  • @user287279, putting the subordinate clause first applies it to both clauses whether you put a comma before the coodinating conjunction or not. If you stick an ambiguous subordinate clause at the end, the comma won't resolve the ambiguity, but word order or some clarifying unit can. The main thing is: don't put it last position. – KarlG Apr 10 '18 at 16:59
  • @user287279: As for dropping the comma before the conjunction after one after an introductory subordinate clause, other sources retain it: evergreen.edu/sites/default/files/writingcenter/handouts/… – KarlG Apr 10 '18 at 17:09
  • Thanks again KarlG for the comment and the link. I think I'll have to study more about this issue because there seems to be conflicting advice on the net. – user287279 Apr 11 '18 at 2:54
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According to A Writer's Reference, 7th Ed., by Diane Hacker and Nancy Sommers, c.2011, p259:

The comma...Use a comma before a coordinating conjunction joining independent clauses...Exception: If the two clauses are short and there is no danger of misreading, the comma may be omitted.

  • The plane took off and we were on our way.

Now, to answer your question about the compound sentences with independent clauses sharing a subordinate clause, this is how I would punctuate them:

  • I swim every day and my friend plays tennis five times a week, because exercise is good.

  • I always feel fresh but John always feels somber, whenever it rains.

So, do exempt the comma from between the two independent clauses. But then use the comma to set your subordinate clause apart from the main clauses of your compound sentence.

And according to Purdue Owl online, there are exceptions to the rule regarding comma use before subordinate clauses:

Don't put a comma after the main clause when a dependent (subordinate) clause follows it (except for cases of extreme contrast).

Also:

Use commas wherever necessary to prevent possible confusion or misreading.

  • 2
    Thank you very much for your opinions and the reference. There seem to be different opinions regarding this matter. Perhaps, it's a matter of style? Anyway, IMHO, I think if punctuations are for making sentences clearer in their meanings, then one should use them when they do so and not use them when they do otherwise. I think the last two examples in your post seem to suggest this principle too. – user287279 Apr 11 '18 at 6:42

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