Knob-and-tube wiring
An early electrical wiring method found in older homes that often needs updating.
Knob-and-tube is a wiring system used in homes built before roughly the 1950s, named for the ceramic knobs and tubes that hold and protect the wires. It isn't inherently dangerous when undisturbed, but it has no ground, can't safely handle modern electrical loads, and is often damaged by later insulation or amateur repairs. Many insurers won't cover homes that still have active knob-and-tube, so it's a key thing to ask about when buying an older home.