use of "comma And" & "AND" while connecting 2 noun modifiers

[rakesh.anfield]

I would really appreciate if if any of the experts can clear my below doubt.


My issue is when i have 2 noun modifiers modifying a noun and which are separated by AND.

the below sentence is from Manhattan Sentence correction - Guide 8.

Tobacco companies, WHICH have been shaken by a string of legal setbacks in the United States
, but WHICH retain strong growth prospects in the developing world, face an uncertain future

why is there a comma before the AND??


"which have been shaken.." and "which retain..." are parallel relative clauses, so what is the need for the ",AND" wouldn't only AND suffice??

i thought of the solution is a way that the first relative clause is a NON ESSENTIAL modifier, which starts off with a comma and the comma belonging to the Comma AND is just to close off this non essential modifier.

but isn't


WHICH have been shaken by a string of legal setbacks in the United States
but
WHICH retain strong growth prospects in the developing


a single huge non essential modifier???

so why use a "comma AND"



I found the below on a topic discussed a few years back, regarding the use of "CommaAnd" & "And" between 2 verb modifiers
I am ok with this.

modifiers-using-ing-t2083.html

but when it comes to noun modifiers i would need some help.

~Rakesh

[jlucero]

Tobacco companies, WHICH have been shaken by a string of legal setbacks in the United States
, but WHICH retain strong growth prospects in the developing world, face an uncertain future

why is there a comma before the AND??

I'll assume you mean "but" in this sentence here. First and foremost, let me explain that the GMAT will not test you on your knowledge of comma usage. Again, you do not need to know the rules of commas, so I wouldn't spend much time learning this. However, the short explanation is you include a comma between conjunctions IF you are joining two full, independent clauses.

She likes to run marathons and surf in the ocean.
She likes to run marathons in her free time, and she also likes to surf large waves in the ocean.

If the clauses are short, you can get away with not including the comma. But there isn't a definitive rule here:

She likes to run and she likes to surf.

In the example you gave, you have two longer clauses, so the extra comma, even as a part of a longer, subordinate clause, is acceptable here.