A note on British titles of rank, which many students find puzzling, may be helpful. The five ranks of the peerage are, in ascending order, baron, viscount, earl, marquess (the French spelling "marquis" is not seldom used), and duke. Peers of the peerages of England, Great Britain (creations since 1707), and the United Kingdom (creations since 1801) automatically become members of the House of Lords. Members of the peerage of Ireland, before the abolition of the Irish parliament in 1801, were entitled to sit in the Irish House of Lords in Dublin, but not in the British House of Lords; they might, however be elected to the British House of Commons--Viscount Palmerston, the nineteenth-century Prime Minister, an Irish peer, sat in the Commons as member for an English constituency. The Scottish peers, at the beginning of each new Parliament, elect sixteen of their number to represent them in the British House of Lords--the Earl of Bute, George III's Prime Minister, sat in the Lords as a Scottish representative peer--but the remainder are not eligible to the House of Commons. Many Scottish and Irish peers, however, also hold subordinate English (or, after 1707, British) peerages, in right of which they sit in the British House of Lords: e.g., the Earl of Shelburne (peerage of Ireland) sat n the House of Lords as Baron Wycombe (peerage of Great Britain).
Baronets (hereditary) and knights (non-hereditary) are not peers, but commoners; they are designated "Sir," with given and family name (as "Sir Robert Walpole"). Wives of peers are, in order, baronesses, viscountesses, countesses, marchionesses, and duchesses. Wives of baronets and knights are "Lady" with the family name only: Maria Skerret, who married Sir Robert Walpole, became "Lady Walpole."
Much confusion is caused by "courtesy titles" and the designations "Lord" and "Lady," none of which are official styles of any peer. Eldest surviving sons of earls, marquesses, and dukes are given the "courtesy" title of (usually) the second highest peerage held by the father: e.g., the eldest son of the Duke of Chandos was referred to as the Marquess of Carnarvon (but in official documents merely "Henry Brydges, Esquire, commonly styled Marquess of Carnarvon"). Younger sons of marquesses and dukes bear the courtesy designation "Lord" with given and family names: Lord Sidney Beauclerk, father of Johnson's friend Topham Beauclerk, was fifth son of the Duke of St. Albans. "Lord" with the title only, and without "of," is also the normal designation, except on official occasions, of barons (it would be very unusual to hear a baron addressed as "Baron So-and-so"), and is the informal mode of address of viscounts, earls, and marquesses, but not dukes: e.g., the Marquess of Rockingham was frequently referred to as "Lord Rockingham," but the Duke of Grafton never as "Lord Grafton."
Baronesses (normally) and viscountesses, countesses, and marchionesses (informally) are addressed as "Lady," with the husband's title, but never duchesses. All daughters of earls, marquesses, and dukes are "Lady" with given and family names. When married to a man of lower rank, they change their own family name to their husband's, but retain their own given name: e.g., when Lady Mary Pierrepont, daughter of Evelyn Pierrepont, Duke of Kingston, married Mr. Edward Wortley Montagu, she became Lady Mary Wortley Montagu (her husband remained "Mr. Wortley Montagu"). Marrying a man of higher rank, they assume the title his wife would normally carry: e.g., when Lady Diana Spencer, daughter of Charles Spencer, Duke of Marlborough, married Lord Bolingbroke (2nd Viscount Bolingbroke), she became Viscountess Bolingbroke (Lady Bolingbroke). Divorced from him and married to Topham Beauclerk, she became Lady Diana Beauclerk (Topham remained "Mr. Beauclerk"). The wife of the younger son of a marquess or duke, if of lower rank than her husband, becomes "Lady" with her husband's given and family names: Topham Beauclerk's mother was Lady Sidney Beauclerk. Children of peers not entitled to a courtesy "Lord" or "Lady" are "the Honourable" Mr., Miss, or Mrs. So-and-so. But "Right Honourable" is the attribution of members of the King's Privy Council (e.g., the Rt. Hon. Joseph Addison, PC, MP) and of barons, viscounts, and earls (marquesses are "Most Honourable" and dukes "Most Noble"). Dukes, duchesses, and archbishops (but not archbishops' wives) are formally addressed as "Your Grace"; other peers and peeresses and bishops (but not bishops' wives) as "Your Lordship" or "Your Ladyship" (or "My Lord" or "My Lady"). In the Church of England, archbishops are "Most Reverend," bishops "Right Reverend," deans "Very Reverend," and archdeacons "Venerable."
Holders of courtesy titles are not peers but commoners, and may, if elected, sit in the House of Commons: e.g., Lord George Gordon, MP, third son of the Duke of Gordon, and Lord (i.e., Baron--by courtesy) North, MP, eldest son of the Earl of Guilford ("MP," like "Congressman," indicates a member of the lower house of the legislature). However, the heir to a peerage might be summoned (by the Crown) to the House of Lords in one of his father's junior peerages: e.g., John Hervey, second son of the Earl of Bristol, became on the death of his older brother Carr Hervey in 1723, heir to the earldom; he automatically received the courtesy title Lord (i.e., Baron) Hervey, and was elected to the House of Commons; in 1733, he was given a writ of summons to the House of Lords in his father's peerage of Baron Hervey of Ickworth (i.e., his title was no longer a courtesy but a substantive one, and from a commoner he had become a peer, although his usual designation of "Lord Hervey" remained unchanged).
The title "Lord," with the name of a landed estate, used by Scottish judges has nothing to do with any peerage; it is not hereditary and it does not confer the epithet "Honourable" on the children of its holders (though Lord Auchinleck's eldest son, James Boswell, used it to impress people on the Continent, who presumably knew no better). Nor does "Lord" in the names of various offices of government (Lord High Chancellor, Lord Keeper of the Privy Seal, Lord Commissioner of the Treasury or Admiralty) have any connection with the peerage, and a Mr. Jones appointed to one of them remains Mr. Jones still.
The numbering of the holders of a peerage begins again each time the title is conferred anew. Thus Aubrey de Vere, 20th and last Earl of Oxford (of the De Vere family) died in 1703; in 1711, Robert Harley was created 1st Earl of Oxford (of the second creation); in the nineteenth century the succession to the earldom died out in the Harley family, and in the 1920s Herbert Henry Asquith was again created 1st Earl of Oxford (of the third creation).