Twin-lead cable is a two-conductor flat cable used as a transmission line to carry radio frequency (RF) signals. It is constructed of two multistranded copper or copperclad steel wires, held a precise distance apart by a plastic (usually polyethylene) ribbon. The uniform spacing of the wires is the key to the cable's function as a parallel transmission line; any abrupt changes in spacing would reflect radio frequency power back toward the source. The plastic also covers and insulates the wires. In 300 ohm twin-lead, the most common type, the wire is usually 20 or 22 gauge, about 7.5 mm (0.30 inches) apart. Twin lead has lower signal losses than coaxial cable, the main competing form of transmission line, but is more vulnerable to interference.