Knob-and-tube wiring was an early standardized method of electrical wiring. Wires protected by insulating tubes run through holes drilled in the wooden structures of buildings, supported by porcelain knobs. The knobs and tubes separate the black (hot) and white (neutral) wires.
Most homes that were built prior to 1950 used a type of wiring called knob-and-tube, which as of late last year has basically been blacklisted by traditional insurance companies. So if you are buying a home with knob-and-tube wiring, you will need a secondary insurance company and the rates can be triple the price of a traditional insurance company. Definitely something to watch for when considering buying a home.
Here are a few informative articles about knob-and-tube wiring: