That there are five orders, ranks or degrees in the Peerage - barons, viscounts, earls, marquesses and dukes - is common knowledge. What seems sometimes overlooked is that there are also five classes of peers. They are:
1. Peerages of England - created before the Union with Scotland on 1 May 1707;
2. Peerages of Scotland - created before the Union with England;
3. Peerages of Ireland - whether created before the Union with Great Britain or since;
4. Peerages of Great Britain - created after the Union with Scotland and before the Union with Ireland;
5. Peerages of the United Kingdom of Great Britain and Ireland - created since the Union with Ireland on 2 July 1800.
All male holders of peerages in categories 1, 4 and 5, are also Peers of Parliament, and on accession are entitled to seats in the House of Lords  - unless debarred by any of the established disqualifications. These are: infancy, felony, bankruptcy, or being of alien nationality.
 Since Valentine Heywood wrote this in 1951 there have been three major pieces of legislation affecting the Peerage.
Firstly, there was the Life Peerages Act 1958, which put on a statutory basis the undoubted power of the Crown to create life peerages, and secondly there was the Peerage Act 1963, which covered three areas: it authorised the disclaimer for life of certain hereditary peerages; it included among the peers qualified to sit in the House of Lords all peers in the Peerage of Scotland and peeresses in their own right in the Peerages of England, Scotland, Great Britain and the United Kingdom; and finally it removed certain disqualifications of peers in the Peerage of Ireland in relation to the House of Commons and elections thereto.
Thirdly, and perhaps most importantly, there has been the House of Lords Act 1999 which has removed the automatic right of hereditary peers to a seat in the House of Lords. As a quid pro quo for the hereditary peers agreeing to the removal of this ancient right of theirs, provision was made in the Act for the right to sit and vote in the House of Lords to be retained for the Earl Marshal (the Duke of Norfolk) and the Lord Great Chamberlain (the Marquess of Cholmondeley) as well as 90 hereditary peers, who were to be chosen by their fellow hereditary peers by way of secret ballot.
Consequently, Heywood's statement in his last paragraph is now totally inaccurate. The present (February 2003) position is that the only Peers of Parliament (omitting the 26 Lords Spiritual) are life peers created under the Appellate Jurisdiction Act 1876 (28) or the Life Peerage Act 1958 (552), the Duke of Norfolk, the Marquess of Cholmondeley, and the 90 hereditary peers elected under the provisions of the House of Lords Act 1999 - all in all some 672 people.