A group switching centre (GSC) was a United Kingdom telephone exchange used to provide local and trunk routing to local exchanges within its own and if required, adjacent group areas. Group switching centres formed a component of the trunk mechanisation hierarchy that was commenced in 1939. The concept of group switching centres became important with the introduction of subscriber trunk dialling (STD) in 1958, initially from Bristol central telephone exchange. They were typically located towards the centre of their corresponding group area, usually in a large town or city, and serviced a group of local and dependent telephone exchanges within their home and adjacent group areas. From 1 January 1958, the British network was divided into 639 charging groups. These areas provided the basis fo