Knob and tube
electrical of wiring has been installed in homes from the 1920s right up into the 1970's in some jurisdictions. Only a hot and neutral wire are provided. The circuit has no electrical ground path. The individual electrical wires are wrapped in a rubberized cloth. That
was fine when the wires remained suspended in air. Wherever the knob and tube circuit wires pass through building framing
lumber a ceramic tube is used to insulate the wire from the wood. Where the knob and tube wiring is surface mounted in a building it is attached using a ceramic "knob". These ceramic knobs are a good insulator provided they have not been damaged or modified. Knob and tube electrical circuits are not "illegal"
and there is not a code requirement that they be replaced. However this wiring method is considered obsolete.