An electrical cable is an insulated electric conductor used to carry power or signals. In its simplest form, a cable has a core of conducting metal (e.g., copper), usually several wires stranded together, surrounded by an insulating sheath of plastic or rubber. Cable for VHF signals, as used in radio, is coaxial cable. Here a central core of wire is surrounded by insulation. For electric power, simple cables are used in the home. Overhead cables, used to transmit power from power stations, are insulated from the pylons that support them, but between pylons there is generally no insulation around the core. Cables with a number of cores (multi-core cables) are used for telephone cables, although these are increasingly being replaced by optical cables.