如果由關係代名詞 who 或 which 所引導的關係子句 (即形容詞子句) 所提供的資訊，對於它所修飾的人或事物是必要的，那麼 who 或 which 的前面不可有逗點，而這種關係子句就叫做「限定關係子句」(restrictive relative clause)，簡稱限定子句 (restrictive clause)，此時 who 或 which 可用 that 來替代。
相對地，如果 who 或 which 所引導的關係子句所提供的資訊，對於它所修飾的人或事物並非必要，而是補充的或額外的資訊，那麼 who 或 which 的前面須有逗點，而這種關係子句就叫做「非限定關係子句」(non-restrictive relative clause)，簡稱非限定子句 (non-restrictive clause)，此時 who 或 which 不可用 that 來替代；申言之，that 所引導的關係子句前面絕對不可有逗點。
例句：
- The man who came to dinner yesterday was John. (昨天來吃晚餐的男子是約翰) (正) － who came to dinner yesterday 是個限定子句，因為它是說明 the man 的必要資訊，所以前後都不可有逗點。
- The man that came to dinner yesterday was John. (正) － 在限定子句中，who 可用 that 來代替。
注意：在口語和非正式寫作中，用 that 來指人仍可以接受，但在正式寫作中，應避免這麼做。
- The bicycle which was stolen the other day was found. (前幾天被偷的腳踏車找到了) (正) － which was stolen the other day 是個限定子句，因為它是說明 the bicycle 的必要資訊，所以前後都不可有逗點。
- The bicycle that was stolen the other day was found. (正) － 在限定子句中，which 可用 that 來代替。
- Cindy has always been close to both her parents, who live in the same village. (辛蒂一直跟她住在同一個村子的父母親很親) (正) － who live in the same village 是個非限定子句，因為它只是 her parents 的補充資訊，所以子句前後要有逗點。在非限定子句中，who 不可用 that 來代替
- My house, which I bought last year, has been sold. (我去年買的房子已賣掉) (正) － which I bought last year 是個非限定子句，因為它只是 My house 的補充資訊，所以子句前後要有逗點。在非限定子句中，which 不可用 that 來代替。
- The girl, who lives next door, has gone to France. (住隔壁的女孩已經去法國) (誤) － who lives next door 是個限定子句，因為它是說明 the girl 的必要資訊，所以前後都不可有逗點。
- The car, which Mary drove on Thursday, has been stolen. (瑪麗星期四開的那部車已經遭竊) (誤) － which Mary drove on Thursday 是個限定子句，因為它是說明 the car 的必要資訊，所以前後都不可有逗點。
- His youngest daughter who was born on 16 June 1996 went to Japan three days ago. (他最小的女兒三天前去日本。她生於 1996 年 6 月 16 日) (誤) － who was born on 16 June 1996 是個非限定子句，因為它只是 his youngest daughter 的補充資訊，所以子句前後都要有逗點。
- Allen is my dream guy. Allen who is a pastry chef is getting married to another girl the day after tomorrow. (艾倫是我的夢中情人。艾倫是個糕餅師傅，他後天就要跟另一位女孩結婚) (誤) － who is a pastry chef 是個非限定子句，因為它只是 Allen 的補充資訊，所以子句前後都要有逗點。
- The man who doesn’t read good books has no advantage over the man who can’t read them. (不讀好書的人不會比不能讀好書的人來得強) (馬克‧吐溫 Mark Twain) (正)
- It is only shallow people who do not judge by appearances. (只有膚淺的人才不會以貌取人) (王爾德 Oscar Wilde) (正)
- One should never trust a woman who tells one her real age. A woman who would tell one that would tell one anything. (絕對不要相信會透露自己真實年齡的女人。這種女人什麼都說得出來) (王爾德) (正)
- Experience is a comb which nature gives us when we are bald. (經驗是在我們禿頭時大自然給我們的一把梳子) (正)
然而，關係子句並不是「非黑即白」，在同一句中只能有單一用途，亦即只能用作限定子句或非限定子句。事實上，有時在同一句子中，它們既可當限定子句，亦可當非限定子句，而且兩者皆合乎文法，惟意思有所不同。例如：
- Peter has always been close to his brother who works for the same company. (彼得一直跟他任職同一家公司的兄弟很親)
- Peter has always been close to his brother, who works for the same company. (彼得一直跟他任職同一家公司的兄弟很親)
在例句 1，一般大多會認為彼得至少有兩個兄弟，而他跟那位任職同一家公司的兄弟很親，而在例句 2，一般大多會認為彼得只有一個兄弟，而他碰巧跟彼得在同一家公司工作。
- Manx cats, which live on the Isle of Man, have a longer life expectancy than normal domestic cats. (曼島貓的平均壽命比一般家貓來得長) － 這句的意思是說，所有曼島貓的平均壽命都比一般家貓來得長。關係子句 which live on the Isle of Man 只是補充資訊，告訴讀者曼島貓的棲息地。曼島 (the Isle of Man) 是位於英格蘭與愛爾蘭之間的一個島嶼。
- Manx cats which live on the Isle of Man have a longer life expectancy than normal domestic cats. (曼島貓的平均壽命比一般家貓來得長) － 這句的意思是說，只有曼島的曼島貓才有比較長的平均壽命，其他地方的曼島貓則沒有。
最後要提的是，在限定子句中，若關代當受詞用，則 who、which 或 that 可以省略。例如：
- The woman who I met yesterday will visit us today. (我昨天遇見的那個女子今天將會拜訪我們)
= The woman that I met yesterday will visit us today.
= The woman I met yesterday will visit us today. - The letter which I received was from my father. (我收到的那封信是我父親寫來的)
= The letter that I received was from my father.
= The letter I received was from my father.