Knights of the shire
The composition of the House of Commons, and the way in which its members were elected, underwent important changes in the 15th and 16th centuries.
The most prominent members in the Commons were the knights of the shire. During the Middle Ages two knights were elected for each of the 37 counties under royal jurisdiction. In 1536 the twelve counties of Wales were incorporated into English rule by statute and they gained the right to return one member each to Parliament. Later two counties long seen as outside royal jurisdiction, the county palatines of Chester and Durham, were also able to return two members each to Parliament, from 1543 and 1673 respectively.
When a new Parliament was summoned, writs were issued from Chancery (the royal secretariat) to the county's sheriff to call a County Court for an election of knights of the shire, and in the early days of Parliament all freemen, that is those who were not serfs, had the right to vote for their representatives.
The 40 shilling franchise
The rules were changed by a statute of 1429 which, finding that elections had recently been crowded by people of "low estate", decreed that only freemen who owned freehold land (that is, not leased from the land's owner) worth 40 shillings had the vote. This restricted the vote to a much smaller group of landowners, and the 40 shilling franchise was only abolished in 1832 by the Great Reform Act.
During this period of aristocratic strife, the Wars of the Roses, the leading peers of the different factions tried to build up the number of their followers in the Commons and they took advantage of the opportunity to restrict the elections, in both voters and candidates, to landowners like themselves. From this point the knights of the shire largely came from and primarily expressed the interests of the landed elite, known as the gentry, and were often brought into Parliament by the influence and efforts of the peers in the Lords.