I was inspecting a vacant house the other day when I came across knob and tube wiring. Anyone who has purchased an older home has probably learned the hard way that the term "knob and tube" ranks right up there with "asbestos" or "vicious dog" as far as homeowner insurance companies are concerned. Most companies don't want to insure houses with it.
If you are in the market for a house and are looking at homes built before 1950 be sure to check out the wiring. If it has knob & tube you will probably need to change it out in order to insure it in the voluntary market.
Here's a link to an article I found that explains what knob & tube is and it's pros and cons: https://www.carsondunlop.com/inspection/blog/knob-and-tube-wiring-home-owners-insurance/