The surest way to fuzzy thinking.        [  ]
How best to baffle a reader.                [  ]
When you really want to be unclear.   [  ]
What's an antecedant, anyway?            [  ]
I like to have my reader working in reverse!    [  ]

    These two guys, this & which, are often used as
relative pronouns.  They're in the family with who:
    who is for persons, which is for things, so is that . . .
. . . you remember that?   This can get in there, too, as can it, though these are not relative pronouns but something called
expletives. Forget that. Think of this as being problematic
right along side which, when used too loosely.

        Bob disliked Amy, who was his sister's nemesis. This puzzled us.
           A nemesis, usually used in the plural nemeses, is a word that we've taken from Green mythology: Nemesis, the goddess of getting even, revenge.
           Mythology, which is a subject I deeply love, is not a great collection in the sky of untruths. That's a limited notion, one that will blind us to the importance of myths, the very stories we live by.
            So those three underlined usages are relative pronouns: who, that, which
and the purple This is an expletive like It, used here to refer back to...to what? To the fact that Bob disliked Amy, or to the fact that Amy was Bob's sister's come-upppance? You can't tell and therein lies the problem.
            But hold on This, and let's (let + us => let ' s ) look at the pronouns:

           They are called "relative" because they relate back to an antecedent.
            An "antecedent" means comes before, and whatever it is that comes before will be either a noun or a pronoun:
            Amy -- who      word -- that        mythology -- which     one -- that
          [Proper noun]                          [common noun]                                  [another noun]                                         [pronoun]

This is why people will pack your mind with cotton, when they use a relative pronoun to refer back not to a noun or a pronoun, but to something much more complicated, which is why it becomes vague, unclear, which is not what you want. This would be bad writing.
        You can see what happens. If you use the relative pronoun to refer back to a whole idea, instead of to a clear antecedent expressed as a noun or pronoun, then you don't know what part of the idea you're refering to and the whole thing becomes vague, blurry. You can feel your mind's eye go crossed. Antidote for cross-eyed minds:
         Which and this must refer to clear antecedents.

I guess a "rule" would be:
            Which should always have a clear antecedent expressed as a noun or a pronoun.
       And for this:  For clarity, always try to have this refer to a noun or a pronoun.

Sometimes you can get away with a vague this if the context is clear:
     Enkidu and Gilgamesh do battle with a giant that is a cannibal and is ultimately defeated by being blinded. This should remind you of the one-eyed giant that Odysseus battles in The Oddyssy.

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