Knob-and-tube wiring is an older method of electrical wiring, frequently used in homes in the early 20th century. The name comes from the porcelain knobs that support the electrical wire and the tube insulators that cover the wire as it passes through studs and joists.
Knob-and-tube wiring that's properly installed and in good condition isn't inherently dangerous, according to the International Association of Certified Home Inspectors. However, it can be more susceptible to fraying and deterioration due to the age of the wiring. It can also pose a greater risk of fire if wall insulation envelops the wiring and the system is under heavy load. Knob-and-tube wiring also lacks grounding wire to safely divert excess electricity.