Since the very beginning in the early 1920s, public broadcasting in the Netherlands has been split into different broadcasting associations with their members composed of listeners and viewers. These associations were based on the different ideological sections of Dutch society, called "Verzuiling" (pillarisation). Catholics, Protestants and Socialists were the first groups to create their own institutions, including schools, hospitals, trades unions and political parties. When radio in the Netherlands started in the 1920s the existing groups quickly created their own broadcasting associations, producing programmes for the primary radio network, Hilversum 1. The first to start was the liberal AVRO, founded as radio broadcaster Hilversumsche Draadlooze Omroep (HDO) by the NSF transmitter factory in Huizen on 8 July 1923. The first regular radio broadcasts started on 21 July 1923. Airtime was rented to the various religious and political radio organisations—the Protestant NCRV, the Roman Catholic KRO, the Socialist VARA and the liberal Protestant VPRO.