Why was Princess Diana forbidden to call Charles by his first name before her engagement? Only when Lady Diana was officially engaged to the Prince was she allowed to call him Charles. Before that, she addressed him as Sir. Charles, meanwhile, addressed the girl exclusively by her first name (although Spencer also had a title, Lady). In the same way, "sir", to Charles addressed and Diana's older sister, Lady Sarah, during their nine-month affair with Charles. This is most likely due to the fact that the Spencer sisters were dating the future monarch. And proximity to the throne suggests appropriate treatment. Thus, the most informal thing that Elizabeth's children and grandchildren could afford when addressing her in public was "ma'am" - a slightly softer version of the strict "mam".
