Sometimes referred to as open wiring, knob and tube was the standard way American homes were wired from roughly the early 1900s until the late 1940s. The design uses individual copper conductors wrapped in cloth-and-rubber insulation, routed through porcelain knobs and tubes — and unlike modern wiring, it has no ground. Decades later, that original insulation grows brittle and flakes away, porcelain hardware cracks, and wires droop and wear thin, leaving live conductors exposed. Decades of DIY splices and modifications frequently make matters worse.