What were/are the rules regarding relative pronouns from c. 1800?

I’ve been reading some rather old literature, often ranging from the 18th Century through to the late 19th Century, and I’m trying to increase my comprehension of the material, at least to the extent that, if I wanted to, I could reliably write in a similar manner and be grammatically correct by the grammar rules of that time. I find that one of the more confusing aspects of this to be the use of relative pronouns, sometimes using which in situations that in wouldn’t be during the mid-20th Century through to the present.

Basically, what I am asking is, in a historical context (i.e. not necessarily standard to our English today), what were the distinctions in use and meaning between that, which, who/whom, etc.?