at which在定语从句相当于where。如Can you remember the place where / at which we spent our winter holiday last year？
at which等于什么
at which中的which是关系代词，指代前面的先行词（即定语从句所修饰的词）rate,它在定语从句中作介词at的宾语。这里的at which=which…at。关系代词在定语从句中作介词宾语时，介词可以提前，即将介词置于关系代词之前，不过关系代词who,that除外。例如：
1. That is the house which / that he once lived in.
= That is the house in which / where he once lived.那就是他曾经住过的房子。
2. The man who / that / whom I will go with is a professor
= The man with whom I will go is a professor.要和我一起去的那个人是一位教授。
3. The tree which / that we often enjoyed the cool air under has been cut down.
= The tree under which we often enjoyed the cool air has been cut down.我们经常在下面乘凉的那棵树已经被砍倒了。
关系副词与“介词+which”的区别
引导定语从句时where与in which有时可互换，有时不能互换，注意以下几点：
当“介词+which”在定语从句中作时间、地点和原因状语时，可以用相应的关系副词when,where和why来替换。
1.when根据情况可以等于in which, at which, on which等。如：
The day when（=on which） she arrived was Thursday.她到的那天是星期四。
2.where根据情况可以等于in which, at which, on which等。如：
This is the hotel where（=in which / at which）they were staying.这就是他们当时住的旅店。
3.关系副词若要换成“介词+which”只能是for which。如：
The reason why（=for which）I came here was to be with my family.我到这里来的原因是要跟我的家里人在一起。
判断which前加什么介词
1、of which相当于定语whose，后加名词，相当于是一个形容词。
eg：There are many books in the shelf, of which/whose covers are old.书架上有许多封面陈旧的书。
2、in which相当于疑问词where，是对地点名词的修饰。
eg：The book（in which/where there are many stories）in his.这本有许多故事的书是他的。
3、on which相当于特殊疑问词when，是对时间的修饰。
eg：I will never forget the best time when/on which we enjoyed ourselves.
4、for which相当于疑问词why，是对原因的修饰。
eg：I don't the reason why/for which he was late.
我不知道他迟到的原因。
5、at which相当于疑问词how，是对方式名词的修饰。
