Signaling System No. 5 (SS5) is a telephone signaling protocol used primarily in the United States for connecting telephone calls. It was developed in the 1960s to facilitate communication between telephone exchanges, allowing for efficient call setup and management. SS5 uses a series of electrical signals to convey information about the call, such as the dialing digits and the status of the line. This system is part of the broader category of signaling systems, which includes other protocols like Signaling System No. 7 (SS7). SS5 was widely used in analog telephone networks before the transition to digital systems, playing a crucial role in the evolution of telecommunications technology.