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?