non-defining relative clause
Definition - A clause that gives information about a noun or noun phrase that is not essential for identifying the subject.
Note: This is in contract to a defining relative clause, which gives information that is necessary for identifying the subject.

Example -

(1) His brother, who is a 911Truther, is a nice guy.
(non-defining: There is evidently only one brother)
(2) The brother who is a 911Truther is a nice guy.
(defining: There is evidently more than one brother, but this one is the only 911Truther.)

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