The Heir and the Spare - the term refers to the first two children of the ruling sovereign or a King/Queen-in-waiting. A Spare acts as a "reserve" in the event an heir-apparent, for some reason, could not ascend the throne, such was the case of King Edward VIII and his younger brother King George VI (father of Queen Elizabeth II). When Edward voluntarily abdicated in 1936 to marry his divorced American lover, Wallis Simpson (divorce people were not allowed to join the royal court before) his younger brother (known as Prince Bertie the Duke of York) took his place as sovereign of the United Kingdom and reigned as George VI for 16 years.
Edward and George's father, King George V, was once "a spare" too. George V, grandson of Queen Victoria, was the former Duke of York and second son of Victoria's heir, King Edward VII, but Edward's first born son and heir, Prince Albert Victor, the Duke of Clarence, died at the age of 28 just weeks after the announcement of his engagement to Princess Mary of Teck. The Duke of York had not only became "heir" but also married his brother's fiancee, Princess Mary of Teck.
The latest heir and spare are the Waleses brothers, Prince William and Prince Harry, children of Prince Charles and the late Princess of Wales, Diana. According to the book "Prince William: the boy who will be King" by Randi Reisfeld, during his teenage years, Prince William was terrified with the idea of becoming King that he confided to his mother that he does not want to take the throne and would take a police job instead (to protect his mother). Though Diana dismissed William's fear as part of his innocence, reports began circulating on Prince Harry's possible accession. Many years later, Princess Diana's instinct proved true as her eldest son gradually became serious with his various trainings as future King of England.
In 2007 he was finally allowed to join his regiment in the war zone, so the young Lieutenant went to Afghanistan discreetly and became a tank commander, this mission was handled with utmost secrecy, the British Armed Forces did not release any reports involving the deportation of Prince Harry. But weeks later, the news was leak to international media that the Prince was serving in Afghanistan, fearing for his safety, his direct superiors, ordered his immediate release from the mission, in reverence to his grandmother, Queen Elizabeth II, the commander in chief of the British Armed forces.
The Prince reportedly hesitated at first, expressing his intention to remain with his troops in the battlefield, but his field commanders insisted to send him back to England, deeply saddened with the order, Prince Harry however relented and was greeted happily at the Heathrow airport by his father, Prince Charles and Prince William. Wills embraced his brother and was later seen carrying Harry's luggage unto the waiting royal car.
The Heir and the Spare are once again reunited in England, taking a good break from various military trainings and off to a well-deserve vacation mostly at Klosters, Switzerland for a skiing holiday. William and Harry frequently seen accompanying each other to various events, may it be sports, official tours or charity campaigns, they are inseparable and would often seen doing joint goodwill projects. They bonded closely and shared a common trait they both inherited from their mother - compassion.
But just like what their paternal great auntie, Princess Margaret, had realized, Prince Harry will always be THREE and William is TWO pronouncing their gap in the line of succession.