既能用where又能用in which的情況
引導定語從句，在定語從句中作地點狀語，從句與先行詞的搭配缺少介詞in時，既能用where又能用in which。
（1）This is the house where/in which he used to live.
這是他過去住的房子。
（2）The old school has now turned into a large modern factory where/in which a lot of coulour TV sets are produced every year.
這所舊學校現在已變成一個大型的現代化工廠，每年生產許多彩色電視機。
只用in which不用where的情況
1.引導定語從句，但在定語從句中不作地點狀語，而是作方式狀語時，只用in which，不用where。
（1）This is the best way in which we can do the work well.
這是我們把工作做好的最好辦法。
2.引導定語從句，介詞in與定語從句中某些詞語構成固定搭配時，只用in which不用where。
（1）He left his work in which he had been very successful.
他離開了他非常成功的工作。