A非限定性关系从句位于确定的名词之后。因此它对这类名词不作限定性描述，仅对该词作进一步说明作为补充而已。它不同于限定性关系从句，在句中并非必不可少，即使省略也不会引起意义上的混乱。它不同于限定性关系从句之处还在于它与被说明的名词之间用逗号隔开。非限定性关系从句中的关系代词绝对不能省略。这种句子结构相当正规，在书面语中用得比口语中多。
B用于非限定性关系从句中的关系代词：
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