The Director telephone system was a development of the Strowger step-by-step (SXS) switching system used in London and five other large cities in the U.K. from the 1920s to the 1960s. A large proportion (ca. 70% to 80%) of telephone traffic in large metropolitan areas is outgoing traffic, and it is distributed over many exchanges. A non-Director SXS exchange system is not suitable for these areas.