Theoretically at least, the royal succession should be straightforward. But in reality nature, fate or more ambitious contenders for the throne often intervene. This is also true for England's long and turbulent history and so we know of a number of men and women destined to wear the crown who never did.
Robert Curthose was the eldest son of William the Conqueror and should have ruled England as Robert I after his father's death. But in the first instance, William did not think England a fit kingdom for his son (or Robert not a fit ruler for his new kingdom), because he bequeathed the island realm to his second son William Rufus, giving Robert the duchy of Normandy.
Like many royal brothers over the years, Robert and William Rufus did not get on and William, the more decisive, invaded Normandy and deposed Robert, who promptly went on crusade to save face.
When he returned William Rufus was dead, and his youngest brother Henry now ruled England as Henry I. Robert resumed his rule in Normandy but proved so inept that the Norman barons soon begged Henry to take over in his brother's place. Henry didn't need asking twice. He in turn invaded Normandy, defeated his brother at the Battle of Tinchebray and imprisoned him in various castles in England until his death, twenty-eight years later, on February 3rd 1134. Robert Curthose is buried in Gloucester Cathedral.
The only legitimate son of Henry I, William should have ruled after his father's death. That he did not was this time not due to sibling rivalry or family feuds, but instead to high spirits, much alcohol and a night-crossing of the English Channel.
William had decided to cross from France to England in the newest and fastest ship of his day, the White Ship. This undertaking called for a celebration and William and his contemporaries did themselves proud. It is said that both the passengers and the crew were drunk on setting out and - challenging the captain to prove the speed of his vessel - they sailed from Barfleur on a falling tide, in the dark. The ship struck a rock and sank - taking all who sailed in her to the depths.
King Henry I worked hard, after his only son's death, to have his daughter Matilda - widow of the German Emperor Henry V - accepted as his heir. But while the English barons agreed to this while King Henry I was alive, once he had died they chose his nephew, Stephen, to be king.
Stephen was a gallant, but hardly an effective king and maybe England would have fared better with Henry's daughter. But that Matilda never became queen is in large part due to her own conduct. Like other royals before her, she was haughty and had a knack for putting men's backs up!
When she invaded England to press her claim, some of the English barons chose to back her, and for close on 20 years Matilda's and Stephen's supporters tore the country to shreds. Both sides came close to triumph at times:
Matilda had great personal courage and determination, but at a time when women were regarded as inferior to men she was fighting an uphill battle. Her overbearing conduct also did not inspire men's devotion and she was said to hoard grudges as other men hoarded coins.
After her success at the battle of Lincoln, she had been declared Lady of the English and preparations were in hand for her coronation, when her arrogant manner so upset the citizens of London, that they drove her from the city.
Being a woman and unable to lead her own forces into battle, Matilda relied on her half-brother, Robert of Gloucester, the eldest of King Henry I's bastard sons. When he died in 1148, Matilda finally gave up on her quest for the English crown, leaving her son Henry to carry on the fight.
How England's fortunes would have turned out, had these individuals ascended to the throne we cannot possibly know. But it's a fun exercise for budding novelists to imagine what might have happened if we'd had a King Robert, or even a Queen Matilda.