The term used in EU legislation to refer to any entity responsible for operating, ensuring the maintenance of and developing the transmission system in a given area, its interconnections with other systems and ensuring the long-term ability of the system to meet reasonable demands for the transmission of electricity (Article 2, EU Electricity Directive 2019 ((EU) 2019/944)). In Great Britain, the main TSO was National Grid Electricity Transmission plc (NGET) which had a combined system operator (SO) and transmission owner (TO) role until 1 April 2019. NGET legally separated its TO and SO roles, with the SO role being transferred to National Grid Electricity System Operator (NGESO) (see Practice note, National Grid: splitting system operator and transmission owner roles). A TSO is sometimes referred to as the national electricity transmission system operator (NETSO) if it is the main TSO for a member state. TSOs in EU member states are subject to additional regulation due to the Third Energy Package and Clean Energy Package. For more information, see Practice notes, EU network codes for electricity: overview and EU energy regulation: Third energy liberalisation reform, as amended by the Clean energy package.