These wires were typically copper, although aluminium has also been used, and were carried in balanced pairs separated by about 25 cm (10") on poles above the ground, and later as twisted pair cables. Modern lines may run underground, and may carry analog or digital signals to the exchange, or may have a device that converts the analog signal to digital for transmission on a carrier system.
In most cases, two copper wires (tip and ring) for each telephone line run from a home or other small building to a local telephone exchange. There is a central junction box for the building where the wires that go to telephone jacks throughout the building and wires that go to the exchange meet and can be connected in different configurations depending upon the subscribed telephone service. The wires between the junction box and the exchange are known as the local loop, and the network of wires going to an exchange, the access network.