Parliament was placed in a strong bargaining position when Richard II's cousin Henry Bolingbroke overthrew the King in 1399 and looked to Parliament to legitimise his shaky claim to the throne and to provide him with money. In 1401 the new King conceded that the Commons did not have to grant taxes until he had adequately addressed their grievances.
Grants of money
Then, the Commons objected so strongly when the King neglected them in his discussions with the Lords about a tax, that in 1407 he formally affirmed the right of the Commons to initiate all grants of money, a right which it has jealously guarded ever since.
Petitions to the Crown
The Commons also acquired an equal role in making laws. From its beginning one of Parliament's roles had been as a forum for the hearing of private petitions - requests for help or favour from the people - submitted to the King and Lords.
Laws and statutes
Increasingly, the Commons became the principal petitioners to this Upper House, submitting common petitions which addressed general problems which could be solved by the King through new laws, known as statutes. Then petitioners began to submit their grievances first to the Commons and, based on these petitions, the Commons wrote draft statutes, known as Bills, to be presented to the Upper House.
In 1414 the Commons successfully insisted to Henry V that the King and Lords should not change the wording of any of the Bills submitted by the Commons without its agreement and that no Bill should become an Act (that is, become statute), without their assent. That the Commons were acting at this time as equal partners with the Lords in making legislation is suggested by a written note on a Bill from that reign, which states that the Commons agreed to the Lords' amendments to it.
By the mid-15th century the Commons was in control of granting supply of money to the King and had gone from petitioners to full partners in the formation of statute, the highest law of the land.