非限定性定语从句
非限定性定语从句和它的先行项之间只有比较松散的联系，它不是先行项的不可缺少的组成部分，而仅仅是对先行项提供一些补充说明，因此，如果省略了一个非限制性关系分句，并不影响先行项的所指意义。
例如： My cousin, who is an engineer, went to Europe last week.
如前所诉，非限定性关系分句和它的先行项之间只有比较松散的关系。这种分句在口语中有停顿，在书写中常用逗号隔开。
1、非限定性关系分句的引导词通常是who,whom,whose(指人）和which（指物）等wh-词。
例如：The Chairman, who spoke first, sat on my right.
His speech, which bored everyone, went on and on.
The Chairman's daughter, whose name is Ann, gave me a patient smile.
关系副词where和when也能引导非限定性关系分句。
例如：
Many of our Welsh people(威尔士人）are going to settle in North Carolina, where land is cheap.
I'm seeing the manager tomorrow, when he will be back from New York.
非限定性关系分句，就其意义来说，在句中有时相当于一个并列分句。
例如：
They had a fine walk too, which had done his liver good.
(=They had a fine walk too, and it had done his liver good.)
2、在非限制性关系分句中，which的先行项还可以是上文整个分句。
例如：He tried to stand on his hands for five minutes, which-as you know-is rather a difficult thing to do.
3、which还能指前面句子的一部分：
I have known men far more able than Bowles, but not half so interesting, which is quite a different thing.
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