Unlike the simplex exchange which became more difficult to operate as the number of subscribers increased, the duplex exchange was able to connect any subscriber from any exchange unit. In addition to the jack for each subscriber in the exchange unit, duplex exchanges also provided duplex jacks for all subscribers in that exchange. This eliminated the need to relay calls to other exchange operators (as was required in a simplex exchange) and enabled more efficient exchange operation. The duplex exchange remained in use over three eras: Meiji, Taisho and Showa.

---