So, Lady Mary Smith is not Lady Smith and vice versa. Lord John Brown in not Lord Brown and vice versa. If Mary Smith marries Lord Brown she is Lady Brown, not Lady Mary. He is the Marquess of Queensbury, or Lord Queensbury, or Queensbury to his familiars, and his wife is the Marchioness of Queensbury or Lady Queensbury. His heir apparent takes his next highest title as a courtesy title.
Details are as for duke.
He will nearly always be earl of something. His wife is the Countess of Cranthorpe or Lady Cranthorpe, and she will sign herself Desdemona Cranthorpe. As with a duke, the earl's heir will take the next lowest title as a courtesy title, and the heir's son, the next again. All details are the same. He will be, for example, Viscount Brummidge, usually known as Lord Brummidge, or just Brummidge.