I included an overview of know and tube below. While we've done quite a lot of electrical work at our house, we still have a bit of active knob and tube wiring. We made certain to have our electrician inspect all visible portions. Speaking from experience, it can be hard to insure a home with knob and tube wiring.
From
Historically, the standards for installing electrical wiring were less stringent in the age of knob-and-tube wiring than they are today. Compared to modern electrical wiring standards, the main shortcomings of knob-and-tube era wiring are: knob-and-tube wiring never included a safety ground conductor; knob-and-tube wiring did not confine switching to the hot conductor; knob-and-tube wiring permitted the use of in-line-splices in walls without using an accessible junction box to contain the splice.
Older homes may have knob-and-tube wiring for all or part of their electrical system. Such wiring systems require replacement and modernization, as it is inadequate for modern levels of power use. Wiring may have been damaged by renovations done in the building, and insulation covering the wires may be brittle due to age or may be damaged by rodents or carelessness (for example, hanging objects off wiring running in accessible areas like basements).