關係代名詞可以搭配適當的介系詞，用來代替疑問副詞。相較於疑問副詞，介係詞搭配關係代名詞可以更明確的標示關係子句與先行詞的關係：
I went to the park where I found the stray dog. → I went to a park in which I found the stray dog.
I remember the day when I met him for the first time. → I remember the day on which I met him for the first time.
「介系詞+關係代名詞用法」的關係代名詞為介系詞的受詞，所以不可以省略。
在問句中，介系詞通常會擺在關係子句結尾。不過在較正式的用法中，介系詞也可以擺在關係代名詞前方：
Which street does he live in? → In which street does he live?
Which organization does he work for? → For which organization does he work?
這項原則同樣適用於直述句：
This is the room (which/ that) he was murdered in. → This is the room in which he was murdered.
當介系詞擺在關係子句結尾時，我們可以用 that 替代 who 或 which，或是直接省略關係代名詞。
不過當介系詞擺在關係代名詞前面時，因為關係代名詞為介系詞的受詞，我們不可以用 that 替代 who 或 which，用 who 代替 whom，或是直接省略關係代名詞。
