call signs were introduced in 1920 and, with minor refinements, exist in the same form today. all stations have an alphanumeric; the defining numeral is followed by two letters to form a call sign that is unique to each station. the numeral defines the state or territory in which the station is sited. originally, the following were used: 2 = new south wales (and originally australian capital territory); 3 = victoria; 4 = queensland; 5 = south australia (and originally northern territory); 6 = western australia; 7 = tasmania. the letters often defined the station ownership (e.g.: 2hd = harry douglas; 3db = druleigh business college; 5cl = central broadcasters limited) or geographic region (e.g.: 3wr = wangaratta; 4mk = mackay; 7ho = hobart), but in other cases the letters had no specific meaning. over the years, the following numerals were added: 1 = australian capital territory (but earlier stations still retain their "2" call sign); 8 = northern territory; 9 = military stations during world war ii, and later for new guinea, and papua – then there's 9mi which doesn't really fit into any category (see below under "mobile stations"); 0 = australian antarctic territory.