Who can declare a new duchy

[pcw27]

Obviously the ruler can,

If a Duke has an extra city can they declare that a new duchy?

Can the Marquis of a city declare that a new duchy?

I am of course speaking mechanically. Naturally these actions would be treasonous in certain circumstances (not that it would stop people from doing it).

[Sypher]

No and No.

Only the Ruler can create a new Duchy.