As I have researched this topic it is rather complex so I will give the reader’s digest version. 
At the time of the Norman Conquest (1066) it has been difficult for me to find any concrete rules regarding the succession to the throne. During the reign ogf the House of Wessex, who were just kings of only Wessex until they began to slowly unify England, the succession dopesn’t seem to have many hard and fast rules. Succession to the throne of Wessex/England was vested in the descendents of King Egbert. However, it was not by primogeniture. There were times when young children of the monarch were passed over in the succession for brothers or uncles of the previous monarch.
Another aspect of the monarchy at this point is the governing council, called the Witan or Witenagemot, which also served in electing the monarchy. Prior to hereditary kingship, which was a later development as families sought to consolidated power, the majority of monarchies were elective…even if that election was limited to one family. To this day historians debate the role of the Witain (even the name itself) but there is evidence that controlling the succession was one of their powers.
In 1066 King Edward the Confessor died without any issue (children) causing one of Enland’s first succession crisis. The legend goes that Edward promised the succession to William the Batsard, Duke of Normandy, a relative by marriage. There was also a co-claim that Harold Godwinson had received a similar promise. When Edward died early that January both men claimed that Edward had promised them the succession. Historians debate the legitimacy of both of those claims. Even in its time there were many conflicting accounts of these alleged promises. They possible were both manufactured by each party.
The truth it seems is that Edward had no power to name his successor and that the power to name the successor was vested in the Witan. They did choose Harold Godwinson, a member of a powerful noble family with connections to the rulers of Denmark. Therfore in the month of January Harold was crowned as Harold II, King of England. William, feeling that his inheritence was stolen from him, mounted an invasion of England. I won’t go into detail with the story as everyone is familiar with it. William invaded England from Normandy and defeated the forces of Harold II at Senlac outside of Hastings in October of that year.
After the defeat of Harold the Witan (including Stigand, Archbishop of Canterbury & Archbishop Ealdred of York) tried to elect Edgar Atheling, the heir to the House of Wessex) as King of England but since military might was on the side of William this was an empty election. William was crowned King of England on Christmas Day of 1066. However, it took a few years to consolidate his rule and bring all of England under his thumb. Although at his coronation William desired to stress his legal right to the throne, Stigand, Archbishop of Canterbury refused to place the crown on William’s head saying “to crown one who was covered with the blood of men and the invader of others’ rights.” Archbishop Ealdred of York was the one who actually placed the crown on his head.
The Witan was the legal body that regulated the succession in 1066. They chose Harld as the legal successor to Edward the Confessor and despite his claims of being the legal heir to the throne William I “the Conqueror” was clearly a usurper in the legal sense. When William came to the throne he abolished the Witan and replaced it with the “king’s court” or Curia Regis. He also took the power to name his successor and this power gradually made England a more hereditary monarchy.
William was not the first King of England although some book make him out to be just that. He did profoundly change England though. The amalgamation of old English and Norman culture forged the modern English culture. Every monarch since the Conquest is a descendent of his. When chronicler’s began numbering the kings of England the reign of William the Conqueror was the starting point.
