This article examines the reasons for developing large pair‐gain digital carrier systems, describes two such systems that were developed for Bell System application—namely, the SLM™ Subscriber Loop Multiplexer System and the SLC™‐40 Subscriber Loop Carrier System—and discusses in detail several circuit designs used in the SLC‐40 System. The SLM system is described and its application history is discussed. Also, the experience gained from the SLM development and its impact on the development of SLC‐40 and the Loop Switching System (lss) is reviewed. A detailed description of the SLC‐40 equipment and system operation is presented next. The introduction of this system into the operating telephone companies is also covered. The final section, which discusses circuit innovations that make the SLC‐40 system possible, includes details on channel unit dc signaling and the multiplexer.