The knob and tube wires were single wire cables. Modern wires are typically two or three wires (black, white and sometimes red) and after the mid-1960s, they also included and insulated ground wire.
The two conductor plus ground cables are used for most normal household circuits. (Remember, conductor is just a fancy word for wire.) The three conductors plus ground cables are typically used for 240 volt appliances and multi-wire branch circuits, including split receptacles.
Hope this helps you to better understand??
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