royalty and nobility

Capitalize a full title and a title before a personal name. Otherwise lowercase king, queen, duke, etc., including a title that is in apposition to a proper name.  

For British titles see Lord. Also consult a source such as Burke's Peerage or Debrett's Peerage.

A sovereign's ruling period is indicated with the abbreviation r. (for Latin regnavit) after his or her name, for example: Charles V (r. 1519-1556). The abbreviation distinguishes the ruling period from the sovereign's lifespan. 

            Queen Elizabeth; Elizabeth the Queen; Elizabeth II, Queen of England; the queen

            The queen opens Parliament next week.

            The former queen lives in quiet retirement. 

            the Queen of Great Britain and Northern Ireland

            Juan Carlos, king of Spain; the king of Spain; the king

            Carl XVI Gustaf

            the kings of France and England

            the former shah of Iran, the shah

            the Earl of Argyll, the fourth Earl of Argyll, the 12th Earl of Argyll, the earls of Argyll, the earl

            Queen Mother

            Prince Philip; the Duke of Edinburgh; the duke

            King Charles 

            the Prince of Wales; the prince

            the Duke of Norfolk; the duke

            the heir apparent

            Emperor Augustus; the emperor Augustus

            His Royal Highness, Her Majesty (capitalize even when standing alone)