Oh yeah! i've just had a Eureka moment. Perhaps it's been taught already, but a penny just dropped with me on my own. And it seems so obvious now.
Any given major or minor chord can only belong to a possible 3 keys! That's it. That's what I just figured out. While I'm at it, I guess there is only one possible key for a diminished chord too.
It's like a monkey swinging from tree to tree - it can only get to a certain tree (key) via a certain branch (chord)!
Seems so simple. I'm really starting to understand this whole web of music better.
I remember seeing a question called 'Why do we need theory' i totally get why we do now.
You don't need to reply, I'm just sounding off and know I might sound daft.