The general administrative body elects some of its own members to sit on the executive board, called the "college van dijkgraaf en heemraden". Except for the chairperson (the "dijkgraaf", see below) these executive board members, called "heemraden" or "hoogheemraden" in Dutch, traditionally represent five types of water users: the local population (residents), industry (factories and industrial buildings), municipalities (urban areas), farmers (agricultural land), and public parks. Each water board is headed by a chair ("dijkgraaf", literally: "dike count", but sometimes called "dike reeve" or "dike warden" in English), an ancient office that dates back to the medieval period. The chair is appointed by the government for a period of six years. The chair presides over the executive board and the general administrative body and has certain ceremonial duties as well. The chair of a water board is at the same level as a mayor in local government and a king's commissioner in provincial government. Typically, a water board’s territory is made up of one or more polders or watersheds. The territory of a water board generally covers several municipalities and may even include areas in two or more provinces. As of 2018, there are 21 water boards in The Netherlands. Anyone who is aged 18 or over and is registered with a local authority in the Netherlands can vote in the Water Authority elections, which are held every four years on the same day as the provincial government elections. The last elections were held on March 20, 2019. Prior to the vote, the local council posts each voter a list of candidates, a ballot paper and details of their local polling stations. Historically, the name "hoogheemraadschap" was used for a large area comprising a number of smaller "waterschappen" within its jurisdiction. "Hoogheemraadschap" was also traditionally the word used for water boards located along the Rijn and the Vecht. The term "waterschap" refers to the jurisdiction or to the administrative body. This also applies to "hoogheemraadschap". In Dutch, the plural of "waterschap" is "waterschappen". The plural of "hoogheemraadschap" is "hoogheemraadschappen". In present-day usage, the official term is "waterschap".