“有一种关系分句(即定语从句)，它既是先行项的后置修饰语(即定语)，同时又是另外一个分句结构的宾语(即宾语从句)”此分句结构就被嵌入在定语从句中，即嵌入式关系分句。
该书举例：In his breast-pocket he had a patch of scarlet, which i suppose was the paper cap serving as a handkerchief.
注意两点：
1)从句which was the paper cap serving as a handkerchief 首先是 patch of scarlet的定语从句。
2)从句which(patch of scarlet) was the paper cap serving as a handkerchief 也是i suppose 的宾语从句，即I suppose (that)the patch of scarlet was the paper cap serving as a handkerchief 。
综上所述，当一个定语从句同时是另一个主谓结构的宾语从句是，就可以写为嵌入式关系分句。