But the City 's official title is still Londonderry . This did not change the name of the city , although the city is coterminous with the district , and in law the city council is also the `` Corporation of Londonderry '' or , more formally , the `` Mayor , Aldermen and Citizens of the City of Londonderry '' . The form `` Londonderry '' is used for the post town by the Royal Mail , however use of Derry will still ensure delivery .