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In the development of dial telephone switching systems two fundamentally different arrangements have been devised for controlling the operations of the switches. In one arrangement the switch at each successive stage is directly responsive to the digit that is being dialed. Systems using this method of operation are called direct dial control systems, an example being the step‐by‐step system as commonly used in the Bell System, hi the other arrangement the dialed information is stored for a short time by centralized control equipment before being used in controlling the switching operations. Systems using the second arrangement are known as common control systems, examples of which are rotary, panel and crossbar. These two arrangements have different economic fields of use, the direct dial control being better suited for the smaller telephone exchanges and the common controls for the larger exchanges, especially those in metropolitan areas. A history of the evolution of these types of switching systems is presented, followed by a discussion of their comparative merits for various fields of use.

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