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Lord

A title for a marquis, earl, viscount, or baron (whether a peer or a peer's eldest son holding his father's second title by courtesy), never used with of:
            
Lord Hartington, Lord Derby, Lord Manvers, Lord Palmerston may be used instead of
            the Marquis of Hartington, the Earl of Derby, Earl Manvers, Viscount Palmerston.

It is prefixed as a courtesy title to the given name, with or without a surname, of a younger son of a duke or marquis:
            Lord Randolph Churchill (the third son of the seventh Duke of Marlborough) or Lord Randolph never as Lord Churchill
            Lord John Russell.


A baron (whether a peer or a peer's eldest son known by the title of his father's barony) is always called by his title of peerage (either a surname or a territorial designation), preceded by Lord:
            Lord Tennyson

If the given name is mentioned, it comes first:
            Alfred, Lord Tennyson
            Thomas, Lord Fairfax


As part of the titles of certain high officials and dignitaries:
            the Lord Mayor of London
           
the Lord Chancellor
           
the Lords of the Admiralty.

As a ceremonial title for any bishop or archbishop of the Church of England:
            Lord Bishop.


For specific British titles consult a source such as Burke's Peerage or Debrett's Peerage.

Context generally determines when the use of titles such as Sir and Lord are appropriate:
            A comment about Paul McCartney's being knighted would probably use Sir Paul,
            but a reference to the rock musician might style him simply Paul McCartney.

See also Lady, ROYALTY AND NOBILITY.