Knob-and-tube wiring
An early wiring method using ceramic knobs and tubes to support single insulated conductors, common in homes built before about 1950.
What is Knob-and-tube wiring?
Knob-and-tube wiring runs the hot and neutral as separate conductors supported on ceramic knobs and passing through ceramic tubes at framing members. It has no equipment grounding conductor, its original rubber or cloth insulation becomes brittle with age, and it was designed to shed heat into open air — which means burying it in later-added insulation is a recognised hazard. Functioning knob-and-tube is not automatically unsafe, but modifications and splices made over the decades usually are, and many insurers decline or surcharge homes that still have active circuits. Inspectors should report where it was observed, whether it appears active, and whether it is in contact with insulation.