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There are many answered questions that address the usage of "which" and "what" on this site—many of them marked as duplicates—and there is even a specific tag for this topic. But I could not find any answer for the following question:

Should I use "which" or "what" or something else if I want to express

A does B which/what causes C.

Therein, "which/what" (the "which" or the "what") should not refer to B but to the act of A doing B (i.e. the predicate of the main clause).

For example see these alternatives:

Bob writes on the blackboard which causes a screeching noise.
Bob writes on the blackboard what causes a screeching noise.

Please note: The noise is caused by the writing not by the blackboard.

Side question: Would the use of a comma be appropriate here?

There is a closely related question, "Do we use “which” or “that” when referring to the preceding main clause as a whole?", that aims on "which" versus "that" as the alternatives and is answered in favor of "which". But maybe "what" would be the better choice here.

Jürgen
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You need to use which. And you need a comma, otherwise the which may be interpreted as introducing a defining or restrictive relative clause with blackboard as its antecedent:

  • Bob writes on the blackboard, which causes a screeching noise.

Swan in Practical English Usage (p495) has a section in relatives with the title: 'which referring to a whole clause':

Which can refer not only to a noun, but also to the whole of a previous clause. Note that what cannot be used in this way:

He got married again a year later, which surprised everybody (NOT...,what surprised everybody).

A final point: we usually write on the blackboard, not to it.

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Shoe
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  • Thank you @Shoe for the prompt and profound answer and the helpful hint on the right preposition when "writing on the blackboard". I edited the question accordingly to avoid distractions. – Jürgen May 07 '19 at 11:07
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    @Jürgen. I assume your mother-tongue is German, which uses was as the relative pronoun in such sentences, was manchmal zu einer fehlerhaften Übersetzung verleitet! – Shoe May 07 '19 at 11:26
  • You are absolutely right: "What" resulted from an unconscious word by word translation from German. On the other hand, "writing on the blackboard" parallels exactly the German choice of the preposition: "Wir schreiben auf die Tafel". (However, we can also use "an (at) die Tafel".) So my abstraction kicked in at the wrong moment. – Jürgen May 07 '19 at 11:47
  • By the way, I like your apt and accurate example of a German "was" clause very much. – Jürgen May 07 '19 at 12:02
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    @Jürgen. That's the problem with language transfer. English has a similar grammar to German, which facilitates learning it for German native speakers. But it also increases the likelihood of negative transfer: https://en.wikipedia.org/wiki/Language_transfer – Shoe May 07 '19 at 12:15