I've picked up bits and pieces. Only the king in Anglo-Saxon kingdoms was refered to as any specific title. He had a wife, and children -- and they were the king's wife, and the king's sons, or the king's daughters -- they would all be athelings, of course, but so were other people. In medieval times, the king was "Your Grace" not "Your Majesty", which was reserved to emperors. "Prince" itself came from Augustus Caesar's "First citizen" -- which sort of modest title is rather commoner in real life than it is in fiction. But going beyond that is fun to find.
Ah, well. In one work-in-progress, a character tells another that the scale of the title is inversely proportionally to the importance of the realm. To be sure, he's the old retainer tellng this to the "lady" of a crucial realm. Still, he's not an entire fool.