Knob and tube, or open wiring, was installed from the initial electrical installations in the early 1900s into the 1940s. It is named for the porcelain fittings that were used to support and run cables through wood structures. If it is left alone and no changes have been made to the original installation, and the insulation is still in good, flexible condition, it may still be a safe installation. However, there are a number of issues you should be aware of. If you have knob and tube wiring in your home, you should have it inspected by a licensed electrician.