 Lord Crawley and his identical twin brother (who dont get along)  tersely acknowledge each other by their first names "Jerry" and  "Dean." This is a reference to Dean Martin (I) (qv) and  Jerry Lewis (I) (qv), who were the most successful comedy team of  the 1950s, yet were known to have had several heated arguments late  in their joint career. They finally split up amidst a great deal of  bitterness in 1956 and didnt speak to each other for twenty years.