中学英语课本里有这样一句：

    And it was one of the few man-made objects on earth that could be seen by the astronauts who landed on the moon.

同一个句子中出现了两个定语从句，它们分别由关系代词that 及who引导。由who引导的第二个定语从句的先行词系第一个定语从句中的astronauts。

定语从句中的一个成分（多为作动词或介词宾语的名词）受到另一个定语从句的限制或说明的现象并不少见。又如：

He wants you to know that anyone who chooses the path he has chosen is sure to have periodic holiday in jail.

他要你明白，任何人选择了他所选择的道路就势必会周期性地到监狱里去度你的“假期”。

在这里，定语从句he has chosen 所修饰的the path 系第一个定语从句中的动词宾语。

Shylock was a hardy man who asked a high rate of interest and showed no mercy to debtors who could not pay.

夏洛克十分厉害，他放高利贷并且对无力还债的人毫无怜悯之心。

定语从句who could not pay所修饰的debtors系第一个定语从句中的介词宾语。

当然，第二个定语从句所限制及说明的不仅仅是一个先行词。如：

Mike is the only teacher I know who writes a good hand. 

迈克是我认识的唯一能写一手好字的教师。

句中who writes a good hand限制的是：the only teacher I know。

Is there anything you want that you have not?

你需要点什么你目前还没有的东西吗？

上句的that you have not 限制anything you want。

有趣的是，两个定语从句可因其限制对象的模棱两可而导致歧义。如：

On the table there was a pencil-case which contained a pen-knife that Mary said was not hers.

句中的that Mary said was not hers可以限制：a. a pen-knife;  b. a pencil-case which contained a pen-knife。此句可有以下汉译：

1. 桌上有一铅笔盒，玛丽说里边的削铅笔刀不是她的。

2. 桌上有一只盛有削铅笔刀的铅笔盒，玛丽说这都不是她的。

最后需一提的是，两个定语从句同时出现时，第二个定语从句未必总是处于“从属”位置和以第一个定语从句中的某一成分为其先行词。请看例句：

They remember to this day the sea-fights in a large basin of water with fleets of paper boats, which he made for them and which he would then set on fire to their great joy.

句中两个which所引导的定语从句皆同时以fleets of paper boats为其先行词。这时，第二个定语从句就不能被称为“定语从句中的定语从句”了。