With Prince Philip’s recent death, Prince Charles has succeeded his father as the 2nd Duke of Edinburgh. This is the third creation of the dukedom, most recently bestowed upon Prince Philip in 1947 as the son-in-law of King George VI, and limited to Philip’s male-line descendants. In 1999, it was announced that Prince Philip’s youngest son, Prince Edward, Earl of Wessex, would follow his father as Duke of Edinburgh when the present title “eventually reverts to the Crown.” With Charles now bearing the title, a fourth creation of the dukedom should ensue on the eventual passing of Queen Elizabeth II, when Prince Charles would succeed as monarch and all his current titles are then available for new creations. However, there are a few extremely unlikely possibilities that would not make Prince Edward the Duke of Edinburgh of the fourth creation quite yet.
Should Prince Charles predecease his mother, his son Prince William would then become the 3rd Duke of Edinburgh. The title would still revert to the Crown upon the Queen’s death, as it would as well should Prince William also predecease the Queen, with Prince George of Cambridge succeeding as the next Duke, the title merging with the Crown upon the Queen’s death. However, in the rare event that Prince Charles, Prince William, and Prince George all predeceased the Queen, then the title of Duke of Edinburgh follows a different succession than the line to the throne, as the next heir (per the 2011 Perth Agreement, which replaced male-preference primogeniture with absolute primogeniture, but only for those born after 28 October 2011) would be Princess Charlotte of Cambridge, while the heir to the Dukedom of Edinburgh would be Charlotte’s younger brother Prince Louis of Cambridge.
All unlikely to ever happen of course, but this scenario would prevent the current creation of the dukedom of Edinburgh from reverting to the crown.
All unlikely to ever happen of course, but this scenario would prevent the current creation of the dukedom of Edinburgh from reverting to the crown. The abbreviated chart below, showing all twenty-two descendants of the Queen, indicates their number in line to the Crown, followed, if applicable, by their number in line to the Dukedom of Edinburgh (all descendants are presented in their birth order from their parent). Of the twenty-one in line to the crown after Prince Charles, only the eight male-line descendants of Prince Philip are also in line to the dukedom.
This has come up within kingdoms held by the same person that followed slightly different succession patterns. From King George I through King George III, the Kings of Great Britain were also Electors of Hanover; the United Kingdom of Great Britain and Ireland was established in 1801, and the Kingdom of Hanover in 1814, after which George III, George IV, and William IV were also Kings of Hanover. Upon William IV’s death without legitimate children, the monarchy of the United Kingdom, which then had male-preference primogeniture, went to his deceased younger brother’s daughter (Queen Victoria, daughter of George III’s fourth son), while the kingdom of Hanover (which had agnatic primogeniture) passed to William IV’s younger brother Ernest Augustus, Duke of Cumberland and Teviotdale.
When the monarchies of England and Scotland, and later the United Kingdom, had male-preference primogeniture, a female heir apparent was always possible (i.e., if the eldest son of a king died before his father and had only daughters), but never occurred, except the unique situation of Queen Anne as the heir apparent of her widowed cousin (and brother-in-law), King William III, who became sole monarch following the death of his wife Mary II. If William III had remarried and had children by a later spouse, those children would be in line to succeed to the throne, but behind Anne and her children, under the specific Act of Settlement of 1701. All other queens were the heirs presumptive before their succession to the throne, as their predecessor on the throne could always have a son who would then “jump the line” in succession.
The issues of heir presumptive versus heir apparent also come up in historical fiction as well. Within the universe of Downton Abbey, Lord Grantham (Robert Crawley, 7th Earl of Grantham) was likely to be succeeded in his title by his grandson George Crawley, son of his daughter Mary Josephine and her late husband, and fourth cousin, Matthew Crawley. George’s path to succeeding his maternal grandfather’s title was only as the next male heir via George’s father’s ancestry. If Lord Grantham remarried and had a son, that son would be heir apparent; if George died and there were no other male Crawley male heirs, the title of Earl of Grantham would become extinct.
 The first creation in 1726 was for Frederick, Prince of Wales, eldest son of King George II. As Frederick predeceased his father, the title passed to his son Prince George, who succeeded his paternal grandfather as King George III, and so the title merged with the crown. The second creation in 1866 was for Prince Alfred, son of Queen Victoria. Alfred’s only son predeceased him, and so this title became extinct upon Alfred’s death in 1900.
 Upon Elizabeth I’s accession to the throne in 1558, the next in line was her first cousin once removed Mary, Queen of Scots, until the latter’s execution in 1587. Mary was always technically heiress presumptive, as any child of Elizabeth I would “jump the line,” even though with time such an event became (as a practical matter) impossible, as Elizabeth I was unmarried and 53 at her cousin’s death.