Equity developed around the 12th century when England was conquered under the rule of William the conqueror. The territory of England was different from the Normandy in terms of the custom and the sociological composition of the people.
Itinerant judges travelled around the land settling disputes in the name of William the conqueror and reported to him about the customs which were similar. Thus what serves as the basis of Common Law are the customs and practices of people which are similar across the land brought together by itinerant judges to form a system of justice.
Writs are documents introduced by the common law judges to recognize the grievances of the party as genuine and something to be dealt with. It is fixed thus it must match the grievance.
However, the inadequacies of remedy caused people to petition their grievances to the king who directed it to the chancellor. The Lord Chancellor was usually a clergy man and as a result ruled according to the dictates of his conscience, good faith and reasoning.
The court of chancery became popular because litigants that were not satisfied with the judgment at the common law courts instituted actions at the court of chancery.
This caused conflict between the common law judges and the Lord Chancellor but got resolved in the Earl vs Oxford case, where it was held that when there was a conflict between the common law court and the court of chancery, the court of chancery prevailed.
The two courts remained separate till the judicature act of 1873 consolidated both courts into the supreme court of judicature.