非限定關係子句會就句中論及的人事物額外補充資訊，但這些資訊並不會指明它談的是何者。就拿這個例句來說：Gorillas, which are large and originate in Africa, can sometimes be found in zoos. 在這個句子中，我們談的是「所有大猩猩」，而不是只有其中幾隻。非限定關係子句會多加補充有關一般大猩猩的資訊，不會限定群體或單一隻大猩猩。如果非限定關係子句從句中移除，剩下的句子文法上仍是正確的，整句話的意思不會有太大的改變，頂多少了些資訊罷了。
- John's mother, who lives in Scotland, has 6 grandchildren.
- My friend John, who went to the same school as me, has just written a best-selling novel.
- My grandmother, who is dead now, came from the North of England.
- We stopped at the museum, which we had never visited before.
- I've just come back from London, where John lives..
- Yesterday I met a woman named Susan, whose husband works in London.
在英語口頭上，限定關係子句常用 that 來取代 who、whom 和 which 等代名詞。但在非限定關係子句中，就不能用 that 來取代其他關係代名詞，也不能像限定關係子句一樣省略關係代名詞。在非限定關係子句中，關係代名詞是不可或缺的，就算關係代名詞是子句中的受格，也不能省略。非限定關係子句一般會用逗號來與主要子句隔開，而限定關係子句通常不用逗號分隔。
- He gave me the letter, which was in a blue envelope.（非限定子句：信只有一封，而且是藍色的；這時您必須用 which。）
- He gave me the letter which/that was in a blue envelope.（限定子句：信不只一封，而且顏色有很多種，而他給我的是藍色的那封信。which 可以被 that 取代，並且不用加逗號。）
- He gave me the letter, which I read immediately.（非限定子句：信只有一封，而 which 是動詞 read 的受詞，但不能省略。）
- Stratford-on-Avon, which many people have written about, is Shakespeare's birthplace.（介系詞一般會放在關係子句的句尾。）
- Stratford-on-Avon, about which many people have written, is Shakespeare's birthplace.（書面用語上常會將介系詞放在代名詞前。）
在關係代名詞前，可以用 all of 或 many of 來引導非限定子句。
|all of, any of, some of, a few of, both of, each of, either of, half of, many of, most of, much of, none of, one of, two of 等||whom||which|
- There were a lot of people at the party, many of whom I had known for years.
- There are 14 girls in my class, a few of whom are my friends.
- He was carrying his belongings, many of which were broken.
- He had thousands of books, most of which he had read.
- He picked up a handful of stones, one of which was sharp.
用 which 來指整個主要子句的資訊
在非限定關係子句上，句首 which 可以指前面句子的所有資訊，而不是單指子句前面的那個字。
- Chris did really well in his exams, which is quite a surprise.
- My friends were all hiding in my apartment, which isn't what I'd expected.
- She's studying to become a doctor, which is difficult.