Hmmm!, Linguistcally Caerdydd is an invention, presumably to provide work for otherwise unemployable Welsh sign writers.

Cardiff = Stem Latin. Caer = Fort or Castle. Diff = By or on the Taff.

Local pronounciation = Kairdif.

But Caerdydd ? Never ! And any Southwalian will tell you so, and we know 'cos we were there !

Gareth