relative pronoun?

Yes, both of them are.

Thank you.
I understand that the that is a relative pronoun.
If so, is it possible to substitute the that for which?
What I found which was interesting was that she is rich.

It is possible but uncommon.

Mister MicawberYes, both of them are.

"What he found that was interesting was that she is rich".

Not the second "that", MM. I think you'll find it is a subordinator introducing the content clause "she is rich". It can't be a relative word because it has no antecedent.

There is other relative word though in the OP's exampl

The "that" phrase is a noun clause. It is similar to an object. It answers the question of "what he found interesting." What did he find interesting? "He found it interesting . . . " WHAT did he find interesting? "He found it interesteing "that she is rich."

DeanbushThe "that" phrase is a noun clause. It is similar to an object. It answers the question of "what he found interesting." What did he find interesting? "He found it interesting . . . " WHAT did he find interesting? "He found it interesteing "that she is rich."

It is actually a clause, not a phrase, functioning as complement to "be" in its specifying sens

BillJhence the preference for the term 'content clause'.

So far, limited to Huddleston & Pullum, I think. Right, Bill?

Not quite - Google it and you'll find a lot of references to it. But the important thing is that adopting a simple term like 'content clause', and dumping names like 'noun clause' and that-clause actually makes it easier for students to understand these clauses.

For example, I can't remember how many times I've been asked why they're called that-clauses when the "that" is so

BillJIf I get time, I'll start a thread on the problems of basing the classification of clauses on analogies with the parts of speech.

Good. We can sticky it in FAQs or somewhere.