This might possibly be my 4th post on peerages. I'm really getting obsessed with the topic, but it fascinates me. Today I did a small piece over on the Authors and Books blog in which I mentioned that Your Majesty came into use at a certain date. Henry VIII introduced it, copying the Kings of France, but it did not become standard form until James I. So all those early medieval romances should not use the address Your Majesty but Your Grace.
There's a great post on this blog here if anyone wants to learn more.