I've been delving into several stories to get some inspiration for speaking to members of a royal family. I've learned a whole lot, but one thing I can't seem to figure out is why in some stories people address kings as "Your Grace" and in others "Your Majesty". So far, I have yet to see anyone use these titles interchangeably, so I am assuming they mean slightly different things? Or am I wrong? Can I use both to create a little variety in my writing?