传统语法的定语从句大致相当于现代语法的关系分句，但是二者也不尽相同。

传统语法认为，当先行词被as, the same, such修饰时，定语从句由as引导，而不用which/that引导。即传统语法认为下面as引导的从句为定语从句：

I have as powerful a car as you have.

This is the same pen as I lost last week.

He is such a teacher as we all like.

现代语法则认为以上三句中的as从句均为比较分句，而不是关系分句。

同样，传统语法认为as可以引导非限制性定语从句，修饰整个主句，这种定语从句可以位于主句前，主句后，也可以插入主句中。例如：

As is known to all, China has the most population in the world.

China has the most population in the world, as is known to all.

China, as is known to all, has the most population in the world.

现代语法则认为以上的as从句为比较分句，而不是关系分句。

现代语法认为，融合型关系代词what=that which, the thing which, 其引导的从句称为名词性关系分句，为关系分句的一种。而传统语法则不认为这种分句是定语从句，而是名词性从句。例如：

What he said was a lie.

因此，传统语法的定语从句和现代语法的关系分句不同完全划等号。

传统语法使用定语从句这个名称有一个致命的缺陷，即先行词不是名词的时候仍称其为定语从句，显然违反逻辑，属于概念混乱。例如which从句修饰主句内容时，以及that从句修饰副词最高级时：

He won the game, which surprised everyone. which从句并不是定语，却叫做定语从句。

He was running the fastest that he had ever run. that从句修饰副词最高级，不可能是定语，但传统语法却只能称其为限制性定语从句。

现代语法则认为这二句中的which从句和that从句都是关系分句，第一句中，which引导关系分句作补充语（补充性状语）；第二句中that引导限制性关系分句作副词的修饰语。

仅从定语从句和关系分句这一点来说孰优孰劣，则关系分句的名称显然优于定语从句的名称。但语法是个体系，牵涉很多定义和概念，传统语法也难以仅仅将定语从句简单的改为关系分句，只好沿用定语从句这个不够严谨准确的名称。