Maybe JA did not tell us what Lizzy will call Mr Darcy as first names were considered very intimate in the early Nineteenth-Century. Just as JA describes little about physical chareceristics of her people. I notice in Ch.31 Aunt Catherine addresses her nephew as Fitzwilliam but I doubt anyone else would be so formal. His best friend Bingley refers to him as Darcy and I think his cousin Fitzwilliam calls him Darcy as well. Afterall, Fitzwilliam is quite a mouthful. I doubt she'd shorten it to 'William' or 'Will.' Maybe 'Fitzwilliam!' if she were cross. I think she'd follow the custom of his intimates and call him Darcy. LOL! Wonder what Mrs Bennet will call Darcy. Son! ;)