作用：用来提供附加而非必要信息，只是对先行词作进一步的解释、补充或说明。若去掉此定语从句，整个主句意义仍然很明确，不会引起误解和歧义。
例句
a. Beijing, which is the capital of China, has developed into an international city.
b. My mother, who is 50 this year, lives with me now.
c. Einstein, who was a great Jewish scientist, came up with the theory of relativity.
d. Hawaii, which consists of eight principal islands, is a favorite vacation spot.
以上各句中的定语从句都是在对其前面的先行词进行补充说明，向读者提供附加信息，均不能变为限制性定语从句。比如，若说My mother who is 50 this year lives with me now.则让人理解为“我有好几位母亲，目前和我住在一起的是今年50岁的那位。”
总的原则：如果定语从句修饰一个专有名词，我们一般要用逗号隔开，也就是说要用非限制性定语从句。这是因为专有名词本身意思已经很完整，不需要限制，定语从句只是对它进行补充。