Modern democratic states with bicameral parliamentary systems are sometimes equipped with a senate, often distinguished from an ordinary parallel lower house, known variously as the "House of Representatives", "House of Commons", "Chamber of Deputies", "National Assembly", "Legislative Assembly", or "House of Assembly", by electoral rules. This may include minimum age required for voters and candidates, proportional or majoritarian or plurality system, and an electoral basis or "collegium". Typically, the senate is referred to as the upper house and has a smaller membership than the lower house. In some federal states senates also exist at the subnational level. In the United States, every state has a senate, with the exception of Nebraska (whose legislature is a unicameral body called the "Legislature" but whose members refer to themselves as "senators"). There is also the US Senate at the federal level. Similarly in Argentina, in addition to the Senate at federal level, eight of the country's provinces, Buenos Aires, Catamarca, Corrientes, Entre Ríos, Mendoza, Salta, San Luis (since 1987) and Santa Fe, have bicameral legislatures with a Senate. Córdoba and Tucumán changed to unicameral systems in 2001 and 2003 respectively.