Heirs apparent who never acceded

Wilhelm, German Crown Prince, Crown Prince of Prussia (Friedrich Wilhelm Victor August Ernst; 6 May 1882 – 20 July 1951) was the eldest child of the last Kaiser, Wilhelm II, German Emperor, and his consort Augusta Victoria of Schleswig-Holstein, and thus a great-grandson of Queen Victoria, and distant cousin to many British Royals, such as Queen Elizabeth II. As Emperor Wilhelm's heir, he was the last Crown Prince of the German Empire and the Kingdom of Prussia, until the abolition of the monarchy.
Wilhelm became crown prince at the age of six in 1888, when his grandfather Frederick III died and his father became emperor. He was crown prince for 30 years until the fall of the empire on 9 November 1918. During World War I, he commanded the 5th Army from 1914 to 1916 and was commander of the Army Group German Crown Prince for the remainder of the war. After his return to Germany in 1923, he fought the Weimar Republic and campaigned for the reintroduction of the monarchy in Germany. After his plans to become president had been blocked by his father, Wilhelm supported Adolf Hitler's rise to power, but when Wilhelm realised that Hitler had no intention of restoring the monarchy, their relationship cooled. Wilhelm became head of the House of Hohenzollern on 4 June 1941 following the death of his father and held the position until his own death on 20 July 1951.

Lieutenant Colonel Geoffrey Charles Tasker Keyes, (18 May 1917 – 18 November 1941) was a British Army officer of the Second World War and a recipient of the Victoria Cross, the highest award that can be made to British and Commonwealth forces for gallantry in the face of the enemy. At the time he was the youngest acting lieutenant colonel in the British Army.

Princess Marie-Louise Razafinkeriefo of Madagascar (1 May 1897 – 18 January 1948) was the last heir apparent to the throne of the Kingdom of Madagascar. She was a grandniece, and the adoptive daughter, of Ranavalona III. During World War II, she worked as a nurse and was made a Dame of the National Order of the Legion of Honour by the French government for her medical service.

John Churchill, Marquess of Blandford (13 February 1686 – 20 February 1703) (sometimes called Charles Churchill) was a British nobleman. He was the heir apparent to the Dukedom of Marlborough – as the only surviving son of John Churchill, 1st Duke of Marlborough, an accomplished general, and Sarah Churchill, Duchess of Marlborough, a close friend of Queen Anne. Blandford died childless in 1703, and upon his father's death in 1722, the dukedom passed to his eldest sister, Lady Henrietta Godolphin (née Churchill).

Edgar Stuart, Duke of Cambridge (14 September 1667 – 8 June 1671) was the fourth son of James, Duke of York (later James II of England) and his first wife Anne Hyde. He was second in the line of succession to the English and Scottish thrones.

Henry, Duke of Cornwall (1 January 1511 – 22 February 1511) was the first living child of King Henry VIII of England and his first wife, Catherine of Aragon, and though his birth was celebrated as that of the heir apparent, he died within weeks. His death and the failure of Henry VIII and Catherine to produce another surviving male heir led to succession and marriage crises that affected the relationship between the English church and Roman Catholicism, giving rise to the English Reformation.