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I came across the following sentence in Sapiens by Yuval Noah Harari:

Even prisons and concentration camps are cooperation networks, and can function only because thousands of strangers somehow manage to coordinate their actions.

I've always read that one should use a comma to separate two independent clauses.

However, in the example above, the comma before "and" offsets a dependent clause.

Grammarly goes so far as to say that the above construction is incorrect:

Don’t use a comma before and when one of the clauses it’s connecting is a dependent clause. Example: "Sam tossed the ball, and watched the dog chase it."

Meanwhile, Purdue OWL states:

Use a pair of commas in the middle of a sentence to set off clauses, phrases, and words that are not essential to the meaning of the sentence.

Thus, I'm assuming that the latter half of Harari's sentence is parenetical, hence the comma usage. Or does another grammatical rule govern its usage?

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    I'm sorry you've always read that, but unfortunately that's not the rule for how to use commas. There are a lot of ways, and a lot of rules, and most of the rules are wrong and/or incomplete, because they're just repeating what somebody said in school. Commas are put in where you hear them. If you don't hear them, say it out loud and listen. Commented Aug 29, 2022 at 19:59
  • Let's start with trusting Purdue, and our ear, over Grammarly - correct sometimes, a nuisance sometimes. Forget parenthetical as not applying in your sentence. The comma there breaks up a longish sentence to read better. That's all. Commented Aug 29, 2022 at 22:16
  • @JohnLawler: Let's not replace a simplification with another simplification, shall we. Commented Aug 30, 2022 at 3:50
  • Rather than being two separate clauses ("Sheep are woolly and eat grass"), the part after the comma follows logically from the first part, and the comma represents where a speaker would pause to indicate entailment. Grammarly cannot understand the semantic or rhetorical structure of a sentence.
    – Stuart F
    Commented Aug 30, 2022 at 10:09

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A coördinating conjunction introducing a non-independent clause is not normally preceded by a comma. This rule you mention is often repeated, and it is overall not a bad rule, but, like most rules, there are exceptions to it.

An afterthought is one such exception: whenever part of a sentence feels like an afterthought, it can be marked off by a comma. I would say the second clause in your sentence bears some resemblance to an afterthought: the first clause says that prisons are coöperation networks, a fairly complete thought. What follows after and is loosely connected to the first clause, an elaboration of sorts: not the parallel thought you'd expect after a standard and.

Another reason why one sometimes places a comma here can be length. If otherwise the sentence would be very long, a comma can be used to give the reader some room to breathe. Your sentence is not extremely long, but it is longish. I would still only do this if the conexion between the two clauses is not very strong.

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  • I don't think the section after the comma in the example is an afterthought.
    – Stuart F
    Commented Aug 30, 2022 at 10:10
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    @StuartF Not your decision to make. Commas are placed by writers, not readers, because writers know what they're writing, and presumably what they want it to sound like. Readers hafta figure this out. Commented Aug 30, 2022 at 15:03
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    Another reason for the comma is to prevent garden path parsing. "X are Y and" seems like the beginning of "X are Y and Z". The comma makes it clear that you're starting a new clause, not continuing the noun phrase.
    – Barmar
    Commented Aug 31, 2022 at 0:17
  • @Barmar: That is a good point. I wouldn't say that was the reason in this sentence, though? Commented Aug 31, 2022 at 0:53
  • I think it's a little of each of these things. It's not quite an after thought, but it's independent enough that it makes sense to demarcate it.
    – Barmar
    Commented Aug 31, 2022 at 0:56

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