Why is it « en quoi » not « ce en quoi »?

Question

What I've been taught is that if some sort of verb took a word like avec or sur instead of the usual à and de, I would use the construction ce (insert) quoi.

For example, since it's être d'accord avec quelqu'un sur quelque chose, if I wanted to say "what I agree with you on is..." I'd say « Ce sur quoi je suis d'accord avec vous est... ».

So my question is, why is it for consister en quelque chose, in all the examples I find, there is no ce? For example to say "I don't know what that consists of" you'd say « Je ne sais pas en quoi ça consiste » instead of « Je ne sais pas ce en quoi ça consiste ».

So could someone explain the difference which may or may not be confined to this example I'm not sure.

Answer 1

It all depends on what you do with that group of words rather than how it's constructed in itself.

In the first case you use it as a subjet of être (ce sur quoi […] est) and therefore it requires to be in nominal form whereas in the other sentence you use it after savoir which most of the time in these types of constructions is followed by an indirect interrogative clause rather than a nominal group.

See this related question for more details. The preposition is different but the same principle applies.

Answer 2

As said in an answer "It all depends on what you do with that group"

We can talk about something like emphasis to make sure the other got us or simply to have a "good french"

To summarize

The ce is absolutely useless in this context

Remark

"It is what we're talking about" has a mandatory Ce " C'est ce dont nous parlons"