Knob and tube wiring
Knob and tube wiring (abbreviated K&T) was an early type of electrical wiring. It was commonly used from the 1880s to the early 1940s, and is occasionally, but very rarely, still used today. In knob and tube wiring, wires were held to the wooden structure by ceramic knobs, and were protected by ceramic tubes when they went through a piece of wood. Wires were held to the knobs by another piece of non-conducting wire. A material called loom protected the wire when it entered a junction box, which usually contained an outlet, switch, or light fixture. Splices were soldered together and wrapped in cloth electrical tape. The insulation on knob and tube wiring was usually cotton cloth soaked in asphalt. To prevent short circuits, the hot and neutral wires were kept far apart. This is not necessary on modern wiring because modern insulation alone is enough to prevent short circuits. Knob and tube wiring did not have a ground wire and therefore did not have three-prong outlets. Most electrical codes allow the use of a GFCI to update knob and tube outlets to three-prong without installing any new wire.