The Lords sitting in the medieval House of Lords were of two types, the lords spiritual and the lords temporal.
The lords spiritual were the bishops and abbots. Not many abbots, the heads of religious houses, were ever summoned to Parliament and most who were never attended. After Henry VIII abolished all the monasteries between 1536 and 1539 these posts no longer even existed. But the two archbishops and 19 bishops, later increased to 24 bishops, were all summoned to every Parliament from 1305 until they were excluded from Parliament in 1642, only to be restored there in 1661. They still sit in the House today.
In the early Middle Ages the lords temporal consisted of only a small number of earls and a much larger number of barons, of whom only about a third were summoned to any individual Parliament. The first reference to the nobility as peers comes from 1321 and suggests that already by that time they saw themselves as a coherent group, accountable only to each other. By the middle of the 15th century the lords had been further divided into five ranks, in descending order: dukes, marquesses, earls, viscounts, and barons.
The War of the Roses
From 1422, when the infant Henry VI came to the throne, the realm descended into civil war, the Wars of the Roses, as rival groups of great nobles and their followers fought for power. These peers, at the height of their influence, were able to exclude the many non-noble officials - judges and royal administrators - who previously had often been part of the King's Great Council from voting in the House of Lords.
Lords and noble status
The peers also insisted that a summons to the Lords was an honour due to their noble status, not just an expression of the King's will, which should be transferable to their heirs in perpetuity. By the time Henry VII claimed the crown in 1485 at the Battle of Bosworth Field, a House of Lords consisting exclusively of the lords spiritual and every member of all five ranks of the hereditary peerage, was well in existence.