Hello everybody!
I was wondering if 7ths, 6ths... chords belong to any key? I guess no because they're made out of simple chords with some other notes? And something belongs to some key, than it consists of 3 notes that are in the key, an depending on the space between, it's major, minor or half dim...
Ok, now does any 7th or whatever then sound good if it's origin is in the key?
I know I'm a bit difficult to understand

.
Like:
C is in key of C, would then C7 sound good in the key?
Dm is in key of C, would Dm7 sound good too?
Thanks for help,
Felix