Cherrybim suggests the same thing that those informative pages in the back of my dictionary suggest.
In the introduction to one of her wonderful and entertaining books on etiquette, Miss Manners gives herself a chance to answer a question that she has always waited for someone to ask, but no one has ever asked.
How would Charlemagne, the Holy Roman Emperor, address himself when talking to himself. The answer, of course, is “ma majeste'”