What it is
Knob-and-tube wiring runs single conductors through ceramic knobs and tubes in the framing. It was the standard way to wire a home from the late 1800s until around the 1940s, and many Dallas homes from that era still have some of it, even after renovations.
Why it's a concern
The system was designed for a handful of lights and small appliances. It has no ground wire, so it can't safely support three-prong outlets without added protection. Its rubber and cloth insulation becomes brittle with age.
It was also designed to shed heat into open air, so it can run hot when buried under modern attic insulation. And over the decades, many homes have had unsafe splices added to it.
Insurance and selling
Many insurers decline to cover homes with active knob-and-tube wiring, or charge more to. It also comes up on nearly every home inspection, which can complicate a sale.
Your options
A full rewire replaces every circuit with modern, grounded wiring, and usually takes 3 to 5 days in a typical home. A partial rewire replaces only the circuits that remain, if an earlier renovation already replaced some.
Either way, the work should be permitted and inspected, so you have documentation for insurers and future buyers.