Knob-and-tube wiring is an old type of electrical wiring that was common in San Francisco and other parts of the United States from the late 1800s to the mid-1900s. It is made up of two insulated copper wires that are held in place by porcelain knobs and tubes. The knobs and tubes are attached to wooden studs or joists, and the wires run through holes that have been drilled in the wood. The black wire is the hot wire, and the white wire is the neutral wire. The two wires are separated by the knobs and tubes to prevent them from touching each other.
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