Please help me with commas in the following sentence:

As an example of the successful use of such a model [,] one can mention SymPy computer algebra system [,] which uses Python as a main user interface.

Is it necessary to write the comma after model? I am not sure whether we can consider "As an example of the successful use of such a model" as an introductory phrase. Also, I am not sure about the second comma.


A comma is required after model because, although the phrase comes at the beginning of the sentence, it is a weak interruption. That is to say, you would still have a viable sentence if you removed it.

You need the second comma, because what follows is a supplementary, not an integrated, relative clause.

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  • +1, but I think there second could be optional dependant on context. Tell me if you agree with my answer. – Jon Hanna Feb 3 '13 at 14:48
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    @Jon Hanna. For the relative clause to be integrated rather than supplementary there would have to be more than one SymPy computer algebra system. – Barrie England Feb 3 '13 at 15:38
  • Right you are. I asked because something made me lack confidence in my answer, and that's exactly how I should have been thinking. – Jon Hanna Feb 3 '13 at 16:05
  • Integrated and supplementary, by the way, are the terms used in 'The Cambridge Grammar of the English Language' in place of the more usual defining / non-defining or restrictive / non-restrictive. – Barrie England Feb 3 '13 at 16:07
  • Funny, I half-suspect that was what led my thinking astray, though were I more used to them in such a context, they probably wouldn't have. – Jon Hanna Feb 3 '13 at 16:13

As far as relative pronouns go, a little brown book insists on the comma after "which" because it usually introduces nonessential elements. It opens a relative clause that has more weight than "that."

Companion website to the Little, Brown Book

The exercises on this site are excellent!

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  • (9th edition, section 28c). – livresque Feb 3 '13 at 15:43
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    More than a few allow which in either case though. But +1 because thinking about whether that could serve or would be plain wrong can help tell the cases apart, and would have saved me from the answer I deleted as incorrect. – Jon Hanna Feb 3 '13 at 16:08

Both commas are ok, as they are used to indicate where the reader/speaker might pause for a moment, whether reading aloud or silently.

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    But then that just gives us the question "are these places where the reader/speaker might pause for a moment?" While it can be a good intuitive guide to someone fluent in the language to think "would I pause", it doesn't really tell us anything, and it can lead us astray. (I'll admit that it is how I decide this in practice while actually writing, but I do find it sometimes leaves me with some strangely placed commas when I come back to what I've written). – Jon Hanna Feb 3 '13 at 14:47

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