When are you supposed to use "what" vs. "which" as a relative pronoun in a relative clause?

According to Purdue's Online Writing Lab:

The most common relative pronouns are who/whom, whoever/whomever, whose, that, and which. (Please note that in certain situations, "what," "when," and "where" can function as relative pronouns.)

But it doesn't mention when you should use "what" in that particular role.

Some other examples are:

Where did you buy the dress what you wore last week?

You're that smart banker what killed his wife.

There was an ESL quiz from Singapore where "what" filled in the blank in the following sentence (and "which" was explicitly marked incorrect by the teacher):

This orchestra, ______ these musicians are from, is very good.

According to "First Book in English Grammar" in 1868:

The pronoun what is a relative, when equivalent to that which; when not, it is an interrogative.

Did this use of the pronoun "what" become "officially" ungrammatical at some point?

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    Answered adequately at Restrictive vs Non-restrictive Relative Clause. Note that a relative determiner is exemplified by 'which' in 'The Freedonia Philharmonic, which orchestra these musicians are from, is very good.' – Edwin Ashworth Apr 26 '18 at 15:57
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    This is not about relative determinatives, but relative pronouns. You are quite right: "which" is the obvious choice here. Though it's not a choice here, the subordinator "that" would also be possible. "Which" and "what" can be relative determiners, but not in your example which already has a determiner, "these". – BillJ Apr 26 '18 at 17:44
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    Why is everyone so eager to close the question rather than answer it. If there was an answer already on the site, someone would likely have been able to link to it. In fact, I'd likely have found it already. Why not just answer the question? It's going to be pretty frustrating if you close the question and I still don't have an answer because Stack Exchange is usually my last resort. What shall I do when you take away my last resort. :) – D. Patrick Apr 26 '18 at 19:18
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    @EdwinAshworth, did you not notice that the word "what" does not exist at all in the text of the question you said adequately answers this question? Do you literally just go around closing questions for fun? It's adequately answered by an answer that doesn't even have the word in it? That's absurd. – D. Patrick Apr 26 '18 at 19:38
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    'What' in 'what little money' is a determiner. // 'What' can certainly be used in embedded questions ('I asked what he wanted') as well as regular questions 'What does he want?' // GingerSoftware claims that ' I saw the shoes what you bought last week on sale for less this week.' is acceptable; I won't be visiting it again. so, in answer to your next comment, 'No'. // The 'duplicate' licensed the choice of 'which', which you originally queried. – Edwin Ashworth Apr 26 '18 at 20:19

I suspect that the quiz was just marked incorrectly: "This orchestra, what these musicians are from, is very good" is certainly not possible in standard English. The use of "what" as a relative pronoun in relative clauses that come directly after an antecedent nominal phrase, e.g. in sentences like "You're that smart banker what killed his wife", certainly exists, but is dialectal and considered non-standard, as mentioned in WS2's answer here.

I think the Purdue Writing Lab's statement that "in certain situations, 'what,' 'when,' and 'where' can function as relative pronouns" refers to "fused relative" constructions. You can find some information on those in the answers to the following questions: Usage of "what", Wh- clauses vs Relative clauses, Ambiguity of "I don't know what you know.", "What might have appalled us when we'd started our trip just a few days ago no longer impressed us much".

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