sentences:  Prince Charles is the longest serving heir-apparent, Duke of Cornwall, and Duke of Rothesay, with a tenure of , and Prince of Wales, with a tenure of .Margaret of Scotland, Countess of Kent was heir presumptive to William I of Scotland, Alexander II and Alexander III for a total of about 43 years, 14 days (her tenure as heir presumptive to Alexander II was also the longest single tenure at ).The longest-serving heir-presumptive of either sex was Robert II, who was heir-presumptive to Robert I and then David II for a total of 46 years, 353 days; he also served the longest single tenure ( as heir-presumptive to David II).