the converter is usually installed in a building called the valve hall. early hvdc systems used mercury-arc valves, but since the mid-1970s, solid state devices such as thyristors have been used. converters using thyristors or mercury-arc valves are known as "line commutated converters". in thyristor-based converters, many thyristors are connected in series to form a thyristor valve, and each converter normally consists of six or twelve thyristor valves. the thyristor valves are usually grouped in pairs or groups of four and can stand on insulators on the floor or hang from insulators from the ceiling.