As far as l know, it is usual to use a comma after a modified noun followed by a non-restrictive relative clause as follows:

The new AIDS treatment, which proved to be highly effective, is extremely expensive.

My question is: Are there special cases that we might ignore using the comma in constructions as the one mentioned above.

  • 1
    It's the comma that marks the relative clause as supplementary (non-restrictive). Removing it would make it an integrated (restrictive) relative. You, Mido Mido, have to decide which semantic type of relative it is before deciding whether or not to use a comma. – BillJ Dec 31 '17 at 18:50

No, there are not. That's because omitting the commas would indicate to readers that you are using "which" to introduce a restrictive clause, basically as a substitute for "that." While "that" cannot be used as a substitute for "which" to create a nonrestrictive clause, "which" can be used as a substitute for "that" to create a restrictive clause (see def. 3 - https://www.dictionary.com/browse/which). As such, omitting the commas expresses that "which" is being used restrictively, not non-restrictively. In grammar, it is the commas that set non-restrictive clauses apart, so leaving them out changes the meaning of the sentence.

Does that mean that nobody ever omits the commas while meaning it to be nonrestrictive? No, it doesn't mean that as there are people who write ungrammatically.

Personally, if I had my druthers, I'd just call it ungrammatical to use "which" to mean "that" and do away with the need for commas around such clauses. I believe that would be extremely practical since the only way in spoken English for a listener to discern whether an ensuing relative clause is restrictive or nonrestrictive is whether the speaker has said "that" or "which," respectively, to introduce it, the written commas generally not reflecting any discernably longer pause in actual speech. Were "which" somehow decreed to be used exclusively for nonrestrictive clauses, never restrictive clauses, there would no longer be any reason to require commas around nonrestrictive relative clauses introduced by "which" and we could spare ourselves a whole lot of commas. Alas, I haven't the authority to issue any such decree, though, so it is what it is.


To a careful reader, your example, with a pair of commas, is a non-defining relative clause. Consequently, it can be removed and the sentence would both still remain grammatically correct and its meaning wouldn't change.

If a sentence contains an adjective clause introduced by the relativizer which, a careful writer would firstly determine whether the subordinate adjective clause is defining or non-defining and would use commas or not accordingly.

The use of the relativizer that is different. Usually we don't use a comma before it.

  • Yes, the writer must decide whether the relative is semantically integrated or supplementary in order to decide whether or not to use a comma. Btw, it is extremely rare to find a supplementary relative with that. In fact they are only marginally acceptable in Standard English. – BillJ Dec 31 '17 at 18:59
  • @Bill J ls there a difference between"The red car which you chose was sold" and " The red car, which you chose, was sold"? – Mido Mido Dec 31 '17 at 20:13
  • @MidoMido Yes, If there was more than red one car and you chose one of them, then the restrictive relative clause is required - the kind without a comma. But if there was only one red car, then the clause is non-restrictive and a comma is required. The relative clause in this case simply adds useful, but non-essential information. – BillJ Jan 1 '18 at 7:41

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