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I read this sentence on an educational website:

Now times have changed and you are ready for situations involving forces in two dimensions.

Shouldn't there be a comma before and, since the subject of the sentence has changed (things vs. you)?

Now times have changed, and you are ready for situations involving forces in two dimensions.

What is the exact comma rule for joining two clauses with and?

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    Duplicate? Not quite. Unlinke english.stackexchange.com/questions/412/…, this question specifically addresses clauses, not merely "items in a list"
    – The English Chicken
    Commented Sep 21, 2011 at 22:47
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    Why has the tag oxford-comma been added? The added comma is not a case of the Oxford/serial comma
    – binaryfunt
    Commented Jan 25, 2018 at 20:12
  • The use of commas has drastically been reduced over the years, and in many ways, writing is more lively and understanding has not been harmed (see?). I feel the comma is truly required only if the word after "and" could be mistaken for part of a phrase with "and" and the word preceding it.
    – user8356
    Commented Oct 12, 2020 at 20:17
  • Yes, there should be a comma before and exactly for the reason you have mentioned.
    – Rusty Core
    Commented Oct 13, 2020 at 6:07

2 Answers 2

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According to some authorities, such as Purdue OWL, yes, a comma should be used before "and" in that sentence:

Use commas to separate independent clauses when they are joined by any of these seven coordinating conjunctions: and, but, for, or, nor, so, yet.

It's a compound sentence, as you identified, and should therefore have a comma.

That being said, a more reasonable guideline, as given at Grammartips.homestead.com, governing comma use is that they, like all punctuation, should be used to reduce or eliminate ambiguity. You can often eliminate the comma

if both independent clauses are quite short, especially if the two clauses are very closely related, and even more so if the subject of both clauses is the same, or

if only the first clause is quite short, especially if the two clauses are very closely related, and even more so if the subject of both clauses is the same.

Here is an example involving two short clauses conjoined with 'but':

John went to the store but he didn't buy anything.

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    Hmmm. I like this answer better than mine. Deleting, and +1.
    – T.E.D.
    Commented Sep 21, 2011 at 21:30
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    All very nicely put. I think the general trend is to use less punctuation these days, and it seems to me you recognise (and delicately endorse) that trend with your "...should be used to reduce...ambiguity. You can often eliminate the comma".
    – FumbleFingers
    Commented Sep 21, 2011 at 22:12
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    T.E.D.'s (deleted) answer adds: << I don't think there really is an "exact rule" [binding rule] that covers this situation and tells you whether a comma is required to be (or not to be) put there. It basically comes down to how the author wants the sentence to read. If they want a slight pause there, they should put in a comma. If not, then it should be left out. >> To which I'd further add that a comma before 'but' has the pragmatic effect of increasing contrast. And a comma before a coordinator where the main clauses are lengthy helps readability.
    – Edwin Ashworth
    Commented Feb 7, 2018 at 9:48
  • For completeness: ProWritingAid addresses the corresponding situation with subordinators: 'If a subordinate clause comes before the clause it is attached to then it should be followed by a comma. You do not need a comma before a subordinate clause if it follows the main clause (except "whereas" and "although".) [I'd add "usually" for these exceptions.]' 'I am taking my umbrella because it might rain.' / 'I am taking my umbrella[,] although it probably won't rain.'
    – Edwin Ashworth
    Commented Sep 27, 2023 at 11:52
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I don't think there really is an "exact rule" [binding rule] that covers this situation and tells you whether a comma is required to be (or not to be) put there. It basically comes down to how the author wants the sentence to read.

If they want a slight pause there, they should put in a comma. If not, then it should be left out. T.E.D. Sep 2011

To which I'd further add that a comma before 'but' has the pragmatic effect of increasing contrast. And a comma before a coordinator where the main clauses are lengthy helps readability. EA Feb 2018

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