commas with non-restrictive elements

Most difficulties with the use of the comma hinge on the distinction between restrictive and non-restrictive sentence elements.

A restrictive word, phrase or clause used as a modifier adds a "restrictive" or defining element that is essential to the meaning of the whole; it should therefore not be separated from the word it modifies by a comma or other mark of punctuation.

• The senators who had objected most strongly to the shift in policy were quick to acknowledge the error in their thinking.

(Meaning: Only some senators objected. Here, the clause “who had objected …” defines which senators were in error—i.e. the ones who objected. The clause is therefore restrictive.)

A non-restrictive element provides incidental or additional information that does not affect the essential meaning; it should be set off by a comma or commas.

• The senators, who had objected most strongly to the shift in policy, were quick to acknowledge the error in their thinking.

(Meaning: All the senators objected; the clause “who had objected …” is merely giving secondary information about all the senators rather than defining a subgroup of senators who were in error.)

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